diff --git a/CHANGES.rst b/CHANGES.rst
index e5279bcdb58..03fb6d57d4f 100644
--- a/CHANGES.rst
+++ b/CHANGES.rst
@@ -60,6 +60,9 @@ Incompatible changes
 * #12597: Change the default of :confval:`show_warning_types`
   from ``False`` to ``True``.
   Patch by Chris Sewell.
+* #12083: Remove support for the old (2008--2010) Sphinx 0.5 and Sphinx 0.6
+  :confval:`intersphinx_mapping` format.
+  Patch by Bénédikt Tran and Adam Turner.
 
 Deprecated
 ----------
diff --git a/doc/usage/extensions/intersphinx.rst b/doc/usage/extensions/intersphinx.rst
index f64b5968029..3212075ebfc 100644
--- a/doc/usage/extensions/intersphinx.rst
+++ b/doc/usage/extensions/intersphinx.rst
@@ -128,28 +128,6 @@ linking:
                   ('../../otherbook/build/html/objects.inv', None)),
       }
 
-   **Old format for this config value**
-
-   .. deprecated:: 6.2
-
-   .. RemovedInSphinx80Warning
-
-   .. caution:: This is the format used before Sphinx 1.0.
-                It is deprecated and will be removed in Sphinx 8.0.
-
-   A dictionary mapping URIs to either ``None`` or an URI.  The keys are the
-   base URI of the foreign Sphinx documentation sets and can be local paths or
-   HTTP URIs.  The values indicate where the inventory file can be found: they
-   can be ``None`` (at the same location as the base URI) or another local or
-   HTTP URI.
-
-   Example:
-
-   .. code-block:: python
-
-      intersphinx_mapping = {'https://docs.python.org/': None}
-
-
 .. confval:: intersphinx_cache_limit
 
    The maximum number of days to cache remote inventories.  The default is
diff --git a/sphinx/environment/__init__.py b/sphinx/environment/__init__.py
index deb6af7975a..4539c1eab6a 100644
--- a/sphinx/environment/__init__.py
+++ b/sphinx/environment/__init__.py
@@ -59,7 +59,7 @@
 
 # This is increased every time an environment attribute is added
 # or changed to properly invalidate pickle files.
-ENV_VERSION = 62
+ENV_VERSION = 63
 
 # config status
 CONFIG_UNSET = -1
diff --git a/sphinx/ext/intersphinx/_load.py b/sphinx/ext/intersphinx/_load.py
index b458d6a7e18..7e1c982cde9 100644
--- a/sphinx/ext/intersphinx/_load.py
+++ b/sphinx/ext/intersphinx/_load.py
@@ -11,6 +11,7 @@
 from urllib.parse import urlsplit, urlunsplit
 
 from sphinx.builders.html import INVENTORY_FILENAME
+from sphinx.errors import ConfigError
 from sphinx.ext.intersphinx._shared import LOGGER, InventoryAdapter
 from sphinx.locale import __
 from sphinx.util import requests
@@ -21,55 +22,123 @@
 
     from sphinx.application import Sphinx
     from sphinx.config import Config
-    from sphinx.ext.intersphinx._shared import InventoryCacheEntry
+    from sphinx.ext.intersphinx._shared import (
+        IntersphinxMapping,
+        InventoryCacheEntry,
+        InventoryLocation,
+        InventoryName,
+        InventoryURI,
+    )
     from sphinx.util.typing import Inventory
 
 
 def normalize_intersphinx_mapping(app: Sphinx, config: Config) -> None:
-    for key, value in config.intersphinx_mapping.copy().items():
+    # URIs should NOT be duplicated, otherwise different builds may use
+    # different project names (and thus, the build are no more reproducible)
+    # depending on which one is inserted last in the cache.
+    seen: dict[InventoryURI, InventoryName] = {}
+
+    errors = 0
+    for name, value in config.intersphinx_mapping.copy().items():
+        # ensure that intersphinx projects are always named
+        if not isinstance(name, str):
+            errors += 1
+            msg = __(
+                'Invalid intersphinx project identifier `%r` in intersphinx_mapping. '
+                'Project identifiers must be non-empty strings.'
+            )
+            LOGGER.error(msg % name)
+            del config.intersphinx_mapping[name]
+            continue
+        if not name:
+            errors += 1
+            msg = __(
+                'Invalid intersphinx project identifier `%r` in intersphinx_mapping. '
+                'Project identifiers must be non-empty strings.'
+            )
+            LOGGER.error(msg % name)
+            del config.intersphinx_mapping[name]
+            continue
+
+        # ensure values are properly formatted
+        if not isinstance(value, (tuple, list)):
+            errors += 1
+            msg = __(
+                'Invalid value `%r` in intersphinx_mapping[%r]. '
+                'Expected a two-element tuple or list.'
+            )
+            LOGGER.error(msg % (value, name))
+            del config.intersphinx_mapping[name]
+            continue
         try:
-            if isinstance(value, (list, tuple)):
-                # new format
-                name, (uri, inv) = key, value
-                if not isinstance(name, str):
-                    LOGGER.warning(__('intersphinx identifier %r is not string. Ignored'),
-                                   name)
-                    config.intersphinx_mapping.pop(key)
-                    continue
+            uri, inv = value
+        except (TypeError, ValueError, Exception):
+            errors += 1
+            msg = __(
+                'Invalid value `%r` in intersphinx_mapping[%r]. '
+                'Values must be a (target URI, inventory locations) pair.'
+            )
+            LOGGER.error(msg % (value, name))
+            del config.intersphinx_mapping[name]
+            continue
+
+        # ensure target URIs are non-empty and unique
+        if not uri or not isinstance(uri, str):
+            errors += 1
+            msg = __('Invalid target URI value `%r` in intersphinx_mapping[%r][0]. '
+                     'Target URIs must be unique non-empty strings.')
+            LOGGER.error(msg % (uri, name))
+            del config.intersphinx_mapping[name]
+            continue
+        if uri in seen:
+            errors += 1
+            msg = __(
+                'Invalid target URI value `%r` in intersphinx_mapping[%r][0]. '
+                'Target URIs must be unique (other instance in intersphinx_mapping[%r]).'
+            )
+            LOGGER.error(msg % (uri, name, seen[uri]))
+            del config.intersphinx_mapping[name]
+            continue
+        seen[uri] = name
+
+        # ensure inventory locations are None or non-empty
+        targets: list[InventoryLocation] = []
+        for target in (inv if isinstance(inv, (tuple, list)) else (inv,)):
+            if target is None or target and isinstance(target, str):
+                targets.append(target)
             else:
-                # old format, no name
-                # xref RemovedInSphinx80Warning
-                name, uri, inv = None, key, value
-                msg = (
-                    "The pre-Sphinx 1.0 'intersphinx_mapping' format is "
-                    'deprecated and will be removed in Sphinx 8. Update to the '
-                    'current format as described in the documentation. '
-                    f"Hint: `intersphinx_mapping = {{'<name>': {(uri, inv)!r}}}`."
-                    'https://www.sphinx-doc.org/en/master/usage/extensions/intersphinx.html#confval-intersphinx_mapping'  # NoQA: E501
+                errors += 1
+                msg = __(
+                    'Invalid inventory location value `%r` in intersphinx_mapping[%r][1]. '
+                    'Inventory locations must be non-empty strings or None.'
                 )
-                LOGGER.warning(msg)
+                LOGGER.error(msg % (target, name))
+                del config.intersphinx_mapping[name]
+                continue
 
-            if not isinstance(inv, tuple):
-                config.intersphinx_mapping[key] = (name, (uri, (inv,)))
-            else:
-                config.intersphinx_mapping[key] = (name, (uri, inv))
-        except Exception as exc:
-            LOGGER.warning(__('Failed to read intersphinx_mapping[%s], ignored: %r'), key, exc)
-            config.intersphinx_mapping.pop(key)
+        config.intersphinx_mapping[name] = (name, (uri, tuple(targets)))
+
+    if errors == 1:
+        msg = __('Invalid `intersphinx_mapping` configuration (1 error).')
+        raise ConfigError(msg)
+    if errors > 1:
+        msg = __('Invalid `intersphinx_mapping` configuration (%s errors).')
+        raise ConfigError(msg % errors)
 
 
 def load_mappings(app: Sphinx) -> None:
-    """Load all intersphinx mappings into the environment."""
+    """Load all intersphinx mappings into the environment.
+
+    The intersphinx mappings are expected to be normalized.
+    """
     now = int(time.time())
     inventories = InventoryAdapter(app.builder.env)
-    intersphinx_cache: dict[str, InventoryCacheEntry] = inventories.cache
+    intersphinx_cache: dict[InventoryURI, InventoryCacheEntry] = inventories.cache
+    intersphinx_mapping: IntersphinxMapping = app.config.intersphinx_mapping
 
     with concurrent.futures.ThreadPoolExecutor() as pool:
         futures = []
-        name: str | None
-        uri: str
-        invs: tuple[str | None, ...]
-        for name, (uri, invs) in app.config.intersphinx_mapping.values():
+        for name, (uri, invs) in intersphinx_mapping.values():
             futures.append(pool.submit(
                 fetch_inventory_group, name, uri, invs, intersphinx_cache, app, now,
             ))
@@ -100,10 +169,10 @@ def load_mappings(app: Sphinx) -> None:
 
 
 def fetch_inventory_group(
-    name: str | None,
-    uri: str,
-    invs: tuple[str | None, ...],
-    cache: dict[str, InventoryCacheEntry],
+    name: InventoryName,
+    uri: InventoryURI,
+    invs: tuple[InventoryLocation, ...],
+    cache: dict[InventoryURI, InventoryCacheEntry],
     app: Sphinx,
     now: int,
 ) -> bool:
@@ -130,7 +199,7 @@ def fetch_inventory_group(
                     return True
         return False
     finally:
-        if failures == []:
+        if not failures:
             pass
         elif len(failures) < len(invs):
             LOGGER.info(__('encountered some issues with some of the inventories,'
@@ -143,7 +212,7 @@ def fetch_inventory_group(
                               'with the following issues:') + '\n' + issues)
 
 
-def fetch_inventory(app: Sphinx, uri: str, inv: str) -> Inventory:
+def fetch_inventory(app: Sphinx, uri: InventoryURI, inv: str) -> Inventory:
     """Fetch, parse and return an intersphinx inventory file."""
     # both *uri* (base URI of the links to generate) and *inv* (actual
     # location of the inventory file) can be local or remote URIs
diff --git a/sphinx/ext/intersphinx/_resolve.py b/sphinx/ext/intersphinx/_resolve.py
index 0a3cc8949d2..730385a7359 100644
--- a/sphinx/ext/intersphinx/_resolve.py
+++ b/sphinx/ext/intersphinx/_resolve.py
@@ -28,10 +28,11 @@
     from sphinx.application import Sphinx
     from sphinx.domains import Domain
     from sphinx.environment import BuildEnvironment
+    from sphinx.ext.intersphinx._shared import InventoryName
     from sphinx.util.typing import Inventory, InventoryItem, RoleFunction
 
 
-def _create_element_from_result(domain: Domain, inv_name: str | None,
+def _create_element_from_result(domain: Domain, inv_name: InventoryName | None,
                                 data: InventoryItem,
                                 node: pending_xref, contnode: TextElement) -> nodes.reference:
     proj, version, uri, dispname = data
@@ -61,7 +62,7 @@ def _create_element_from_result(domain: Domain, inv_name: str | None,
 
 
 def _resolve_reference_in_domain_by_target(
-        inv_name: str | None, inventory: Inventory,
+        inv_name: InventoryName | None, inventory: Inventory,
         domain: Domain, objtypes: Iterable[str],
         target: str,
         node: pending_xref, contnode: TextElement) -> nodes.reference | None:
@@ -100,7 +101,7 @@ def _resolve_reference_in_domain_by_target(
 
 
 def _resolve_reference_in_domain(env: BuildEnvironment,
-                                 inv_name: str | None, inventory: Inventory,
+                                 inv_name: InventoryName | None, inventory: Inventory,
                                  honor_disabled_refs: bool,
                                  domain: Domain, objtypes: Iterable[str],
                                  node: pending_xref, contnode: TextElement,
@@ -142,20 +143,21 @@ def _resolve_reference_in_domain(env: BuildEnvironment,
                                                   full_qualified_name, node, contnode)
 
 
-def _resolve_reference(env: BuildEnvironment, inv_name: str | None, inventory: Inventory,
+def _resolve_reference(env: BuildEnvironment,
+                       inv_name: InventoryName | None, inventory: Inventory,
                        honor_disabled_refs: bool,
                        node: pending_xref, contnode: TextElement) -> nodes.reference | None:
     # disabling should only be done if no inventory is given
     honor_disabled_refs = honor_disabled_refs and inv_name is None
+    intersphinx_disabled_reftypes = env.config.intersphinx_disabled_reftypes
 
-    if honor_disabled_refs and '*' in env.config.intersphinx_disabled_reftypes:
+    if honor_disabled_refs and '*' in intersphinx_disabled_reftypes:
         return None
 
     typ = node['reftype']
     if typ == 'any':
         for domain_name, domain in env.domains.items():
-            if (honor_disabled_refs
-                    and (domain_name + ':*') in env.config.intersphinx_disabled_reftypes):
+            if honor_disabled_refs and f'{domain_name}:*' in intersphinx_disabled_reftypes:
                 continue
             objtypes: Iterable[str] = domain.object_types.keys()
             res = _resolve_reference_in_domain(env, inv_name, inventory,
@@ -170,8 +172,7 @@ def _resolve_reference(env: BuildEnvironment, inv_name: str | None, inventory: I
         if not domain_name:
             # only objects in domains are in the inventory
             return None
-        if (honor_disabled_refs
-                and (domain_name + ':*') in env.config.intersphinx_disabled_reftypes):
+        if honor_disabled_refs and f'{domain_name}:*' in intersphinx_disabled_reftypes:
             return None
         domain = env.get_domain(domain_name)
         objtypes = domain.objtypes_for_role(typ) or ()
@@ -183,12 +184,12 @@ def _resolve_reference(env: BuildEnvironment, inv_name: str | None, inventory: I
                                             node, contnode)
 
 
-def inventory_exists(env: BuildEnvironment, inv_name: str) -> bool:
+def inventory_exists(env: BuildEnvironment, inv_name: InventoryName) -> bool:
     return inv_name in InventoryAdapter(env).named_inventory
 
 
 def resolve_reference_in_inventory(env: BuildEnvironment,
-                                   inv_name: str,
+                                   inv_name: InventoryName,
                                    node: pending_xref, contnode: TextElement,
                                    ) -> nodes.reference | None:
     """Attempt to resolve a missing reference via intersphinx references.
diff --git a/sphinx/ext/intersphinx/_shared.py b/sphinx/ext/intersphinx/_shared.py
index f2f52444b99..fed5844947d 100644
--- a/sphinx/ext/intersphinx/_shared.py
+++ b/sphinx/ext/intersphinx/_shared.py
@@ -2,15 +2,40 @@
 
 from __future__ import annotations
 
-from typing import TYPE_CHECKING, Final, Union
+from typing import TYPE_CHECKING, Final
 
 from sphinx.util import logging
 
 if TYPE_CHECKING:
+    from typing import Optional
+
     from sphinx.environment import BuildEnvironment
     from sphinx.util.typing import Inventory
 
-    InventoryCacheEntry = tuple[Union[str, None], int, Inventory]
+    #: The inventory project URL to which links are resolved.
+    #:
+    #: This value is unique in :confval:`intersphinx_mapping`.
+    InventoryURI = str
+
+    #: The inventory (non-empty) name.
+    #:
+    #: It is unique and in bijection with an inventory remote URL.
+    InventoryName = str
+
+    #: A target (local or remote) containing the inventory data to fetch.
+    #:
+    #: Empty strings are not expected and ``None`` indicates the default
+    #: inventory file name :data:`~sphinx.builder.html.INVENTORY_FILENAME`.
+    InventoryLocation = Optional[str]
+
+    #: Inventory cache entry. The integer field is the cache expiration time.
+    InventoryCacheEntry = tuple[InventoryName, int, Inventory]
+
+    #: The type of :confval:`intersphinx_mapping` *after* normalization.
+    IntersphinxMapping = dict[
+        InventoryName,
+        tuple[InventoryName, tuple[InventoryURI, tuple[InventoryLocation, ...]]],
+    ]
 
 LOGGER: Final[logging.SphinxLoggerAdapter] = logging.getLogger('sphinx.ext.intersphinx')
 
@@ -29,14 +54,13 @@ def __init__(self, env: BuildEnvironment) -> None:
             self.env.intersphinx_named_inventory = {}  # type: ignore[attr-defined]
 
     @property
-    def cache(self) -> dict[str, InventoryCacheEntry]:
+    def cache(self) -> dict[InventoryURI, InventoryCacheEntry]:
         """Intersphinx cache.
 
-        - Key is the URI of the remote inventory
-        - Element one is the key given in the Sphinx intersphinx_mapping
-          configuration value
-        - Element two is a time value for cache invalidation, a float
-        - Element three is the loaded remote inventory, type Inventory
+        - Key is the URI of the remote inventory.
+        - Element one is the key given in the Sphinx :confval:`intersphinx_mapping`.
+        - Element two is a time value for cache invalidation, an integer.
+        - Element three is the loaded remote inventory of type :class:`!Inventory`.
         """
         return self.env.intersphinx_cache  # type: ignore[attr-defined]
 
@@ -45,7 +69,7 @@ def main_inventory(self) -> Inventory:
         return self.env.intersphinx_inventory  # type: ignore[attr-defined]
 
     @property
-    def named_inventory(self) -> dict[str, Inventory]:
+    def named_inventory(self) -> dict[InventoryName, Inventory]:
         return self.env.intersphinx_named_inventory  # type: ignore[attr-defined]
 
     def clear(self) -> None:
diff --git a/sphinx/util/typing.py b/sphinx/util/typing.py
index 20ecb6e6c4d..49d071d9c0f 100644
--- a/sphinx/util/typing.py
+++ b/sphinx/util/typing.py
@@ -127,6 +127,8 @@ def __call__(
     str,  # URL
     str,  # display name
 ]
+
+# referencable role -> (reference name -> inventory item)
 Inventory = dict[str, dict[str, InventoryItem]]
 
 
diff --git a/tests/js/fixtures/cpp/searchindex.js b/tests/js/fixtures/cpp/searchindex.js
index 46f48244741..78e5f761485 100644
--- a/tests/js/fixtures/cpp/searchindex.js
+++ b/tests/js/fixtures/cpp/searchindex.js
@@ -1 +1 @@
-Search.setIndex({"alltitles": {}, "docnames": ["index"], "envversion": {"sphinx": 62, "sphinx.domains.c": 3, "sphinx.domains.changeset": 1, "sphinx.domains.citation": 1, "sphinx.domains.cpp": 9, "sphinx.domains.index": 1, "sphinx.domains.javascript": 3, "sphinx.domains.math": 2, "sphinx.domains.python": 4, "sphinx.domains.rst": 2, "sphinx.domains.std": 2}, "filenames": ["index.rst"], "indexentries": {"sphinx (c++ class)": [[0, "_CPPv46Sphinx", false]]}, "objects": {"": [[0, 0, 1, "_CPPv46Sphinx", "Sphinx"]]}, "objnames": {"0": ["cpp", "class", "C++ class"]}, "objtypes": {"0": "cpp:class"}, "terms": {"The": 0, "becaus": 0, "c": 0, "can": 0, "cardin": 0, "challeng": 0, "charact": 0, "class": 0, "descript": 0, "drop": 0, "engin": 0, "fixtur": 0, "frequent": 0, "gener": 0, "i": 0, "index": 0, "inflat": 0, "mathemat": 0, "occur": 0, "often": 0, "project": 0, "punctuat": 0, "queri": 0, "relat": 0, "sampl": 0, "search": 0, "size": 0, "sphinx": 0, "term": 0, "thei": 0, "thi": 0, "token": 0, "us": 0, "web": 0, "would": 0}, "titles": ["&lt;no title&gt;"], "titleterms": {}})
\ No newline at end of file
+Search.setIndex({"alltitles": {}, "docnames": ["index"], "envversion": {"sphinx": 63, "sphinx.domains.c": 3, "sphinx.domains.changeset": 1, "sphinx.domains.citation": 1, "sphinx.domains.cpp": 9, "sphinx.domains.index": 1, "sphinx.domains.javascript": 3, "sphinx.domains.math": 2, "sphinx.domains.python": 4, "sphinx.domains.rst": 2, "sphinx.domains.std": 2}, "filenames": ["index.rst"], "indexentries": {"sphinx (c++ class)": [[0, "_CPPv46Sphinx", false]]}, "objects": {"": [[0, 0, 1, "_CPPv46Sphinx", "Sphinx"]]}, "objnames": {"0": ["cpp", "class", "C++ class"]}, "objtypes": {"0": "cpp:class"}, "terms": {"The": 0, "becaus": 0, "c": 0, "can": 0, "cardin": 0, "challeng": 0, "charact": 0, "class": 0, "descript": 0, "drop": 0, "engin": 0, "fixtur": 0, "frequent": 0, "gener": 0, "i": 0, "index": 0, "inflat": 0, "mathemat": 0, "occur": 0, "often": 0, "project": 0, "punctuat": 0, "queri": 0, "relat": 0, "sampl": 0, "search": 0, "size": 0, "sphinx": 0, "term": 0, "thei": 0, "thi": 0, "token": 0, "us": 0, "web": 0, "would": 0}, "titles": ["&lt;no title&gt;"], "titleterms": {}})
\ No newline at end of file
diff --git a/tests/js/fixtures/multiterm/searchindex.js b/tests/js/fixtures/multiterm/searchindex.js
index a868eb6bdcb..96b093c5fda 100644
--- a/tests/js/fixtures/multiterm/searchindex.js
+++ b/tests/js/fixtures/multiterm/searchindex.js
@@ -1 +1 @@
-Search.setIndex({"alltitles": {"Main Page": [[0, null]]}, "docnames": ["index"], "envversion": {"sphinx": 62, "sphinx.domains.c": 3, "sphinx.domains.changeset": 1, "sphinx.domains.citation": 1, "sphinx.domains.cpp": 9, "sphinx.domains.index": 1, "sphinx.domains.javascript": 3, "sphinx.domains.math": 2, "sphinx.domains.python": 4, "sphinx.domains.rst": 2, "sphinx.domains.std": 2}, "filenames": ["index.rst"], "indexentries": {}, "objects": {}, "objnames": {}, "objtypes": {}, "terms": {"At": 0, "adjac": 0, "all": 0, "an": 0, "appear": 0, "applic": 0, "ar": 0, "built": 0, "can": 0, "check": 0, "contain": 0, "do": 0, "document": 0, "doesn": 0, "each": 0, "fixtur": 0, "format": 0, "function": 0, "futur": 0, "html": 0, "i": 0, "includ": 0, "match": 0, "messag": 0, "multipl": 0, "multiterm": 0, "order": 0, "other": 0, "output": 0, "perform": 0, "perhap": 0, "phrase": 0, "project": 0, "queri": 0, "requir": 0, "same": 0, "search": 0, "successfulli": 0, "support": 0, "t": 0, "term": 0, "test": 0, "thi": 0, "time": 0, "us": 0, "when": 0, "write": 0}, "titles": ["Main Page"], "titleterms": {"main": 0, "page": 0}})
\ No newline at end of file
+Search.setIndex({"alltitles": {"Main Page": [[0, null]]}, "docnames": ["index"], "envversion": {"sphinx": 63, "sphinx.domains.c": 3, "sphinx.domains.changeset": 1, "sphinx.domains.citation": 1, "sphinx.domains.cpp": 9, "sphinx.domains.index": 1, "sphinx.domains.javascript": 3, "sphinx.domains.math": 2, "sphinx.domains.python": 4, "sphinx.domains.rst": 2, "sphinx.domains.std": 2}, "filenames": ["index.rst"], "indexentries": {}, "objects": {}, "objnames": {}, "objtypes": {}, "terms": {"At": 0, "adjac": 0, "all": 0, "an": 0, "appear": 0, "applic": 0, "ar": 0, "built": 0, "can": 0, "check": 0, "contain": 0, "do": 0, "document": 0, "doesn": 0, "each": 0, "fixtur": 0, "format": 0, "function": 0, "futur": 0, "html": 0, "i": 0, "includ": 0, "match": 0, "messag": 0, "multipl": 0, "multiterm": 0, "order": 0, "other": 0, "output": 0, "perform": 0, "perhap": 0, "phrase": 0, "project": 0, "queri": 0, "requir": 0, "same": 0, "search": 0, "successfulli": 0, "support": 0, "t": 0, "term": 0, "test": 0, "thi": 0, "time": 0, "us": 0, "when": 0, "write": 0}, "titles": ["Main Page"], "titleterms": {"main": 0, "page": 0}})
\ No newline at end of file
diff --git a/tests/js/fixtures/partial/searchindex.js b/tests/js/fixtures/partial/searchindex.js
index 356386af8dd..dc6bc1cddbb 100644
--- a/tests/js/fixtures/partial/searchindex.js
+++ b/tests/js/fixtures/partial/searchindex.js
@@ -1 +1 @@
-Search.setIndex({"alltitles": {"sphinx_utils module": [[0, null]]}, "docnames": ["index"], "envversion": {"sphinx": 62, "sphinx.domains.c": 3, "sphinx.domains.changeset": 1, "sphinx.domains.citation": 1, "sphinx.domains.cpp": 9, "sphinx.domains.index": 1, "sphinx.domains.javascript": 3, "sphinx.domains.math": 2, "sphinx.domains.python": 4, "sphinx.domains.rst": 2, "sphinx.domains.std": 2}, "filenames": ["index.rst"], "indexentries": {}, "objects": {}, "objnames": {}, "objtypes": {}, "terms": {"also": 0, "ar": 0, "built": 0, "confirm": 0, "document": 0, "function": 0, "html": 0, "i": 0, "includ": 0, "input": 0, "javascript": 0, "known": 0, "match": 0, "partial": 0, "possibl": 0, "prefix": 0, "project": 0, "provid": 0, "restructuredtext": 0, "sampl": 0, "search": 0, "should": 0, "thi": 0, "titl": 0, "us": 0, "when": 0}, "titles": ["sphinx_utils module"], "titleterms": {"modul": 0, "sphinx_util": 0}})
\ No newline at end of file
+Search.setIndex({"alltitles": {"sphinx_utils module": [[0, null]]}, "docnames": ["index"], "envversion": {"sphinx": 63, "sphinx.domains.c": 3, "sphinx.domains.changeset": 1, "sphinx.domains.citation": 1, "sphinx.domains.cpp": 9, "sphinx.domains.index": 1, "sphinx.domains.javascript": 3, "sphinx.domains.math": 2, "sphinx.domains.python": 4, "sphinx.domains.rst": 2, "sphinx.domains.std": 2}, "filenames": ["index.rst"], "indexentries": {}, "objects": {}, "objnames": {}, "objtypes": {}, "terms": {"also": 0, "ar": 0, "built": 0, "confirm": 0, "document": 0, "function": 0, "html": 0, "i": 0, "includ": 0, "input": 0, "javascript": 0, "known": 0, "match": 0, "partial": 0, "possibl": 0, "prefix": 0, "project": 0, "provid": 0, "restructuredtext": 0, "sampl": 0, "search": 0, "should": 0, "thi": 0, "titl": 0, "us": 0, "when": 0}, "titles": ["sphinx_utils module"], "titleterms": {"modul": 0, "sphinx_util": 0}})
\ No newline at end of file
diff --git a/tests/js/fixtures/titles/searchindex.js b/tests/js/fixtures/titles/searchindex.js
index 9a229d060bf..f503bf35df9 100644
--- a/tests/js/fixtures/titles/searchindex.js
+++ b/tests/js/fixtures/titles/searchindex.js
@@ -1 +1 @@
-Search.setIndex({"alltitles": {"Main Page": [[0, null]], "Relevance": [[0, "relevance"], [1, null]]}, "docnames": ["index", "relevance"], "envversion": {"sphinx": 62, "sphinx.domains.c": 3, "sphinx.domains.changeset": 1, "sphinx.domains.citation": 1, "sphinx.domains.cpp": 9, "sphinx.domains.index": 1, "sphinx.domains.javascript": 3, "sphinx.domains.math": 2, "sphinx.domains.python": 4, "sphinx.domains.rst": 2, "sphinx.domains.std": 2}, "filenames": ["index.rst", "relevance.rst"], "indexentries": {"example (class in relevance)": [[0, "relevance.Example", false]], "module": [[0, "module-relevance", false]], "relevance": [[0, "module-relevance", false]], "relevance (relevance.example attribute)": [[0, "relevance.Example.relevance", false]]}, "objects": {"": [[0, 0, 0, "-", "relevance"]], "relevance": [[0, 1, 1, "", "Example"]], "relevance.Example": [[0, 2, 1, "", "relevance"]]}, "objnames": {"0": ["py", "module", "Python module"], "1": ["py", "class", "Python class"], "2": ["py", "attribute", "Python attribute"]}, "objtypes": {"0": "py:module", "1": "py:class", "2": "py:attribute"}, "terms": {"": [0, 1], "A": 1, "For": 1, "In": [0, 1], "against": 0, "also": 1, "an": 0, "answer": 0, "appear": 1, "ar": 1, "area": 0, "ask": 0, "attribut": 0, "built": 1, "can": [0, 1], "class": 0, "code": [0, 1], "consid": 1, "contain": 0, "context": 0, "corpu": 1, "could": 1, "demonstr": 0, "describ": 1, "detail": 1, "determin": 1, "docstr": 0, "document": [0, 1], "domain": 1, "engin": 0, "exampl": [0, 1], "extract": 0, "find": 0, "found": 0, "from": 0, "function": 1, "ha": 1, "handl": 0, "happen": 1, "head": 0, "help": 0, "highli": 1, "how": 0, "i": [0, 1], "improv": 0, "inform": 0, "intend": 0, "issu": 1, "itself": 1, "knowledg": 0, "languag": 1, "less": 1, "like": [0, 1], "match": 0, "mention": 1, "name": [0, 1], "object": 0, "one": 1, "onli": 1, "other": 0, "page": 1, "part": 1, "particular": 0, "printf": 1, "program": 1, "project": 0, "queri": [0, 1], "question": 0, "re": 0, "rel": 0, "research": 0, "result": 1, "sai": 0, "same": 1, "score": 0, "search": [0, 1], "seem": 0, "softwar": 1, "some": 1, "sphinx": 0, "straightforward": 1, "subject": 0, "subsect": 0, "term": [0, 1], "test": 0, "text": 0, "than": 1, "thei": 0, "them": 0, "thi": 0, "titl": 0, "user": [0, 1], "we": [0, 1], "when": 0, "whether": 1, "within": 0, "would": 1}, "titles": ["Main Page", "Relevance"], "titleterms": {"main": 0, "page": 0, "relev": [0, 1]}})
\ No newline at end of file
+Search.setIndex({"alltitles": {"Main Page": [[0, null]], "Relevance": [[0, "relevance"], [1, null]]}, "docnames": ["index", "relevance"], "envversion": {"sphinx": 63, "sphinx.domains.c": 3, "sphinx.domains.changeset": 1, "sphinx.domains.citation": 1, "sphinx.domains.cpp": 9, "sphinx.domains.index": 1, "sphinx.domains.javascript": 3, "sphinx.domains.math": 2, "sphinx.domains.python": 4, "sphinx.domains.rst": 2, "sphinx.domains.std": 2}, "filenames": ["index.rst", "relevance.rst"], "indexentries": {"example (class in relevance)": [[0, "relevance.Example", false]], "module": [[0, "module-relevance", false]], "relevance": [[0, "module-relevance", false]], "relevance (relevance.example attribute)": [[0, "relevance.Example.relevance", false]]}, "objects": {"": [[0, 0, 0, "-", "relevance"]], "relevance": [[0, 1, 1, "", "Example"]], "relevance.Example": [[0, 2, 1, "", "relevance"]]}, "objnames": {"0": ["py", "module", "Python module"], "1": ["py", "class", "Python class"], "2": ["py", "attribute", "Python attribute"]}, "objtypes": {"0": "py:module", "1": "py:class", "2": "py:attribute"}, "terms": {"": [0, 1], "A": 1, "For": 1, "In": [0, 1], "against": 0, "also": 1, "an": 0, "answer": 0, "appear": 1, "ar": 1, "area": 0, "ask": 0, "attribut": 0, "built": 1, "can": [0, 1], "class": 0, "code": [0, 1], "consid": 1, "contain": 0, "context": 0, "corpu": 1, "could": 1, "demonstr": 0, "describ": 1, "detail": 1, "determin": 1, "docstr": 0, "document": [0, 1], "domain": 1, "engin": 0, "exampl": [0, 1], "extract": 0, "find": 0, "found": 0, "from": 0, "function": 1, "ha": 1, "handl": 0, "happen": 1, "head": 0, "help": 0, "highli": 1, "how": 0, "i": [0, 1], "improv": 0, "inform": 0, "intend": 0, "issu": 1, "itself": 1, "knowledg": 0, "languag": 1, "less": 1, "like": [0, 1], "match": 0, "mention": 1, "name": [0, 1], "object": 0, "one": 1, "onli": 1, "other": 0, "page": 1, "part": 1, "particular": 0, "printf": 1, "program": 1, "project": 0, "queri": [0, 1], "question": 0, "re": 0, "rel": 0, "research": 0, "result": 1, "sai": 0, "same": 1, "score": 0, "search": [0, 1], "seem": 0, "softwar": 1, "some": 1, "sphinx": 0, "straightforward": 1, "subject": 0, "subsect": 0, "term": [0, 1], "test": 0, "text": 0, "than": 1, "thei": 0, "them": 0, "thi": 0, "titl": 0, "user": [0, 1], "we": [0, 1], "when": 0, "whether": 1, "within": 0, "would": 1}, "titles": ["Main Page", "Relevance"], "titleterms": {"main": 0, "page": 0, "relev": [0, 1]}})
\ No newline at end of file
diff --git a/tests/test_domains/test_domain_c.py b/tests/test_domains/test_domain_c.py
index a8a92cb82ae..43a3e44cc68 100644
--- a/tests/test_domains/test_domain_c.py
+++ b/tests/test_domains/test_domain_c.py
@@ -771,7 +771,7 @@ def test_domain_c_build_intersphinx(tmp_path, app, status, warning):
 _var c:member 1 index.html#c.$ -
 '''))  # NoQA: W291
     app.config.intersphinx_mapping = {
-        'https://localhost/intersphinx/c/': str(inv_file),
+        'local': ('https://localhost/intersphinx/c/', str(inv_file)),
     }
     app.config.intersphinx_cache_limit = 0
     # load the inventory and check if it's done correctly
diff --git a/tests/test_domains/test_domain_cpp.py b/tests/test_domains/test_domain_cpp.py
index d8e612e28fa..0d8f81c656d 100644
--- a/tests/test_domains/test_domain_cpp.py
+++ b/tests/test_domains/test_domain_cpp.py
@@ -1424,7 +1424,7 @@ def test_domain_cpp_build_intersphinx(tmp_path, app, status, warning):
 _var cpp:member 1 index.html#_CPPv44$ -
 '''))  # NoQA: W291
     app.config.intersphinx_mapping = {
-        'https://localhost/intersphinx/cpp/': str(inv_file),
+        'test': ('https://localhost/intersphinx/cpp/', str(inv_file)),
     }
     app.config.intersphinx_cache_limit = 0
     # load the inventory and check if it's done correctly
diff --git a/tests/test_extensions/test_ext_inheritance_diagram.py b/tests/test_extensions/test_ext_inheritance_diagram.py
index 45a5ff0dda9..19423c2c5c1 100644
--- a/tests/test_extensions/test_ext_inheritance_diagram.py
+++ b/tests/test_extensions/test_ext_inheritance_diagram.py
@@ -154,7 +154,7 @@ def test_inheritance_diagram_png_html(tmp_path, app):
     inv_file = tmp_path / 'inventory'
     inv_file.write_bytes(external_inventory)
     app.config.intersphinx_mapping = {
-        'https://example.org': str(inv_file),
+        'example': ('https://example.org', str(inv_file)),
     }
     app.config.intersphinx_cache_limit = 0
     normalize_intersphinx_mapping(app, app.config)
diff --git a/tests/test_extensions/test_ext_intersphinx.py b/tests/test_extensions/test_ext_intersphinx.py
index 599f7c7d1c2..8fd848a184d 100644
--- a/tests/test_extensions/test_ext_intersphinx.py
+++ b/tests/test_extensions/test_ext_intersphinx.py
@@ -1,6 +1,9 @@
 """Test the intersphinx extension."""
 
+from __future__ import annotations
+
 import http.server
+from typing import TYPE_CHECKING
 from unittest import mock
 
 import pytest
@@ -8,6 +11,7 @@
 
 from sphinx import addnodes
 from sphinx.builders.html import INVENTORY_FILENAME
+from sphinx.errors import ConfigError
 from sphinx.ext.intersphinx import (
     fetch_inventory,
     inspect_main,
@@ -26,6 +30,14 @@
 )
 from tests.utils import http_server
 
+if TYPE_CHECKING:
+    from typing import NoReturn
+
+
+class FakeList(list):
+    def __iter__(self) -> NoReturn:
+        raise NotImplementedError
+
 
 def fake_node(domain, type, target, content, **attrs):
     contnode = nodes.emphasis(content, content)
@@ -44,7 +56,8 @@ def reference_check(app, *args, **kwds):
 
 
 def set_config(app, mapping):
-    app.config.intersphinx_mapping = mapping
+    # copy *mapping* so that normalization does not alter it
+    app.config.intersphinx_mapping = mapping.copy()
     app.config.intersphinx_cache_limit = 0
     app.config.intersphinx_disabled_reftypes = []
 
@@ -97,7 +110,7 @@ def test_missing_reference(tmp_path, app, status, warning):
     inv_file = tmp_path / 'inventory'
     inv_file.write_bytes(INVENTORY_V2)
     set_config(app, {
-        'https://docs.python.org/': str(inv_file),
+        'python': ('https://docs.python.org/', str(inv_file)),
         'py3k': ('https://docs.python.org/py3k/', str(inv_file)),
         'py3krel': ('py3k', str(inv_file)),  # relative path
         'py3krelparent': ('../../py3k', str(inv_file)),  # relative path, parent dir
@@ -175,7 +188,7 @@ def test_missing_reference_pydomain(tmp_path, app, status, warning):
     inv_file = tmp_path / 'inventory'
     inv_file.write_bytes(INVENTORY_V2)
     set_config(app, {
-        'https://docs.python.org/': str(inv_file),
+        'python': ('https://docs.python.org/', str(inv_file)),
     })
 
     # load the inventory and check if it's done correctly
@@ -274,7 +287,7 @@ def test_missing_reference_cppdomain(tmp_path, app, status, warning):
     inv_file = tmp_path / 'inventory'
     inv_file.write_bytes(INVENTORY_V2)
     set_config(app, {
-        'https://docs.python.org/': str(inv_file),
+        'python': ('https://docs.python.org/', str(inv_file)),
     })
 
     # load the inventory and check if it's done correctly
@@ -300,7 +313,7 @@ def test_missing_reference_jsdomain(tmp_path, app, status, warning):
     inv_file = tmp_path / 'inventory'
     inv_file.write_bytes(INVENTORY_V2)
     set_config(app, {
-        'https://docs.python.org/': str(inv_file),
+        'python': ('https://docs.python.org/', str(inv_file)),
     })
 
     # load the inventory and check if it's done correctly
@@ -386,7 +399,7 @@ def test_inventory_not_having_version(tmp_path, app, status, warning):
     inv_file = tmp_path / 'inventory'
     inv_file.write_bytes(INVENTORY_V2_NO_VERSION)
     set_config(app, {
-        'https://docs.python.org/': str(inv_file),
+        'python': ('https://docs.python.org/', str(inv_file)),
     })
 
     # load the inventory and check if it's done correctly
@@ -400,29 +413,59 @@ def test_inventory_not_having_version(tmp_path, app, status, warning):
     assert rn[0].astext() == 'Long Module desc'
 
 
-def test_load_mappings_warnings(tmp_path, app, status, warning):
-    """
-    load_mappings issues a warning if new-style mapping
-    identifiers are not string
-    """
-    inv_file = tmp_path / 'inventory'
-    inv_file.write_bytes(INVENTORY_V2)
-    set_config(app, {
-        'https://docs.python.org/': str(inv_file),
-        'py3k': ('https://docs.python.org/py3k/', str(inv_file)),
-        'repoze.workflow': ('https://docs.repoze.org/workflow/', str(inv_file)),
-        'django-taggit': ('https://django-taggit.readthedocs.org/en/latest/',
-                          str(inv_file)),
-        12345: ('https://www.sphinx-doc.org/en/stable/', str(inv_file)),
-    })
-
-    # load the inventory and check if it's done correctly
-    normalize_intersphinx_mapping(app, app.config)
-    load_mappings(app)
-    warnings = warning.getvalue().splitlines()
-    assert len(warnings) == 2
-    assert "The pre-Sphinx 1.0 'intersphinx_mapping' format is " in warnings[0]
-    assert 'intersphinx identifier 12345 is not string. Ignored' in warnings[1]
+def test_normalize_intersphinx_mapping_warnings(app, warning):
+    """Check warnings in :func:`sphinx.ext.intersphinx.normalize_intersphinx_mapping`."""
+    bad_intersphinx_mapping = {
+        # fmt: off
+        '':                 ('789.example', None),     # invalid project name (value)
+        12345:              ('456.example', None),     # invalid project name (type)
+        None:               ('123.example', None),     # invalid project name (type)
+        'https://example/': None,                      # Sphinx 0.5 style value (None)
+        'https://server/':  'inventory',               # Sphinx 0.5 style value (str)
+        'bad-dict-item':    0,                         # invalid dict item type
+        'unpack-except-1':  [0],                       # invalid dict item size (native ValueError)
+        'unpack-except-2':  FakeList(),                # invalid dict item size (custom exception)
+        'bad-uri-type-1':   (123456789, None),         # invalid target URI type
+        'bad-uri-type-2':   (None, None),              # invalid target URI type
+        'bad-uri-value':    ('', None),                # invalid target URI value
+        'good':             ('example.org', None),     # duplicated target URI (good entry)
+        'dedup-good':       ('example.org', None),     # duplicated target URI
+        'bad-location-1':   ('a.example', 1),          # invalid inventory location (single input, bad type)
+        'bad-location-2':   ('b.example', ''),         # invalid inventory location (single input, bad string)
+        'bad-location-3':   ('c.example', [2, 'x']),   # invalid inventory location (sequence input, bad type)
+        'bad-location-4':   ('d.example', ['y', '']),  # invalid inventory location (sequence input, bad string)
+        'good-target-1':    ('e.example', None),       # valid inventory location (None)
+        'good-target-2':    ('f.example', ('x',)),     # valid inventory location (sequence input)
+        # fmt: on
+    }
+    set_config(app, bad_intersphinx_mapping)
+
+    # normalise the inventory and check if it's done correctly
+    with pytest.raises(
+        ConfigError,
+        match=r'^Invalid `intersphinx_mapping` configuration \(16 errors\).$',
+    ):
+        normalize_intersphinx_mapping(app, app.config)
+    warnings = strip_colors(warning.getvalue()).splitlines()
+    assert len(warnings) == len(bad_intersphinx_mapping) - 3
+    assert warnings == [
+        "ERROR: Invalid intersphinx project identifier `''` in intersphinx_mapping. Project identifiers must be non-empty strings.",
+        "ERROR: Invalid intersphinx project identifier `12345` in intersphinx_mapping. Project identifiers must be non-empty strings.",
+        "ERROR: Invalid intersphinx project identifier `None` in intersphinx_mapping. Project identifiers must be non-empty strings.",
+        "ERROR: Invalid value `None` in intersphinx_mapping['https://example/']. Expected a two-element tuple or list.",
+        "ERROR: Invalid value `'inventory'` in intersphinx_mapping['https://server/']. Expected a two-element tuple or list.",
+        "ERROR: Invalid value `0` in intersphinx_mapping['bad-dict-item']. Expected a two-element tuple or list.",
+        "ERROR: Invalid value `[0]` in intersphinx_mapping['unpack-except-1']. Values must be a (target URI, inventory locations) pair.",
+        "ERROR: Invalid value `[]` in intersphinx_mapping['unpack-except-2']. Values must be a (target URI, inventory locations) pair.",
+        "ERROR: Invalid target URI value `123456789` in intersphinx_mapping['bad-uri-type-1'][0]. Target URIs must be unique non-empty strings.",
+        "ERROR: Invalid target URI value `None` in intersphinx_mapping['bad-uri-type-2'][0]. Target URIs must be unique non-empty strings.",
+        "ERROR: Invalid target URI value `''` in intersphinx_mapping['bad-uri-value'][0]. Target URIs must be unique non-empty strings.",
+        "ERROR: Invalid target URI value `'example.org'` in intersphinx_mapping['dedup-good'][0]. Target URIs must be unique (other instance in intersphinx_mapping['good']).",
+        "ERROR: Invalid inventory location value `1` in intersphinx_mapping['bad-location-1'][1]. Inventory locations must be non-empty strings or None.",
+        "ERROR: Invalid inventory location value `''` in intersphinx_mapping['bad-location-2'][1]. Inventory locations must be non-empty strings or None.",
+        "ERROR: Invalid inventory location value `2` in intersphinx_mapping['bad-location-3'][1]. Inventory locations must be non-empty strings or None.",
+        "ERROR: Invalid inventory location value `''` in intersphinx_mapping['bad-location-4'][1]. Inventory locations must be non-empty strings or None."
+    ]
 
 
 def test_load_mappings_fallback(tmp_path, app, status, warning):