diff --git a/gems/abbrev/.rubocop.yml b/gems/abbrev/.rubocop.yml
new file mode 100644
index 000000000..7d0be8ff2
--- /dev/null
+++ b/gems/abbrev/.rubocop.yml
@@ -0,0 +1,19 @@
+# This configuration inherits from /.rubocop.yml.
+# You can configure RBS style of this gem.
+# This file is used on CI. It is configured to automatically
+# make rubocop suggestions on pull requests for this gem.
+# If you do not like the style enforcement, you should remove this file.
+inherit_from: ../../.rubocop.yml
+
+##
+# If you want to customize the style, please consult with the gem reviewers.
+# You can see the list of cops at https://github.com/ksss/rubocop-on-rbs/blob/main/docs/modules/ROOT/pages/cops.adoc
+
+RBS/Layout:
+  Enabled: true
+
+RBS/Lint:
+  Enabled: true
+
+RBS/Style:
+  Enabled: true
diff --git a/gems/abbrev/0.1/_test/test.rb b/gems/abbrev/0.1/_test/test.rb
new file mode 100644
index 000000000..8225211a8
--- /dev/null
+++ b/gems/abbrev/0.1/_test/test.rb
@@ -0,0 +1,10 @@
+# Write Ruby code to test the RBS.
+# It is type checked by `steep check` command.
+
+require "abbrev"
+
+Abbrev.abbrev([])
+Abbrev.abbrev(%w(abbrev))
+Abbrev.abbrev(%w(abbrev), 'abb')
+Abbrev.abbrev(%w(abbrev), /^abb/)
+Abbrev.abbrev(%w(abbrev), nil)
diff --git a/gems/abbrev/0.1/abbrev.rbs b/gems/abbrev/0.1/abbrev.rbs
new file mode 100644
index 000000000..6370794c1
--- /dev/null
+++ b/gems/abbrev/0.1/abbrev.rbs
@@ -0,0 +1,66 @@
+# <!-- rdoc-file=lib/abbrev.rb -->
+# Calculates the set of unambiguous abbreviations for a given set of strings.
+#
+#     require 'abbrev'
+#     require 'pp'
+#
+#     pp Abbrev.abbrev(['ruby'])
+#     #=>  {"ruby"=>"ruby", "rub"=>"ruby", "ru"=>"ruby", "r"=>"ruby"}
+#
+#     pp Abbrev.abbrev(%w{ ruby rules })
+#
+# *Generates:*
+#     { "ruby"  =>  "ruby",
+#       "rub"   =>  "ruby",
+#       "rules" =>  "rules",
+#       "rule"  =>  "rules",
+#       "rul"   =>  "rules" }
+#
+# It also provides an array core extension, Array#abbrev.
+#
+#     pp %w{ summer winter }.abbrev
+#
+# *Generates:*
+#     { "summer"  => "summer",
+#       "summe"   => "summer",
+#       "summ"    => "summer",
+#       "sum"     => "summer",
+#       "su"      => "summer",
+#       "s"       => "summer",
+#       "winter"  => "winter",
+#       "winte"   => "winter",
+#       "wint"    => "winter",
+#       "win"     => "winter",
+#       "wi"      => "winter",
+#       "w"       => "winter" }
+#
+module Abbrev
+  # <!--
+  #   rdoc-file=lib/abbrev.rb
+  #   - abbrev(words, pattern = nil)
+  # -->
+  # Given a set of strings, calculate the set of unambiguous abbreviations for
+  # those strings, and return a hash where the keys are all the possible
+  # abbreviations and the values are the full strings.
+  #
+  # Thus, given `words` is "car" and "cone", the keys pointing to "car" would be
+  # "ca" and "car", while those pointing to "cone" would be "co", "con", and
+  # "cone".
+  #
+  #     require 'abbrev'
+  #
+  #     Abbrev.abbrev(%w{ car cone })
+  #     #=> {"ca"=>"car", "con"=>"cone", "co"=>"cone", "car"=>"car", "cone"=>"cone"}
+  #
+  # The optional `pattern` parameter is a pattern or a string. Only input strings
+  # that match the pattern or start with the string are included in the output
+  # hash.
+  #
+  #     Abbrev.abbrev(%w{car box cone crab}, /b/)
+  #     #=> {"box"=>"box", "bo"=>"box", "b"=>"box", "crab" => "crab"}
+  #
+  #     Abbrev.abbrev(%w{car box cone}, 'ca')
+  #     #=> {"car"=>"car", "ca"=>"car"}
+  #
+  def self?.abbrev: (Array[String], ?String | Regexp | nil) -> Hash[String, String]
+end
diff --git a/gems/abbrev/0.1/array.rbs b/gems/abbrev/0.1/array.rbs
new file mode 100644
index 000000000..5ea1f93ce
--- /dev/null
+++ b/gems/abbrev/0.1/array.rbs
@@ -0,0 +1,26 @@
+%a{annotate:rdoc:skip}
+class Array[unchecked out Elem]
+  # <!--
+  #   rdoc-file=lib/abbrev.rb
+  #   - abbrev(pattern = nil)
+  # -->
+  # Calculates the set of unambiguous abbreviations for the strings in `self`.
+  #
+  #     require 'abbrev'
+  #     %w{ car cone }.abbrev
+  #     #=> {"car"=>"car", "ca"=>"car", "cone"=>"cone", "con"=>"cone", "co"=>"cone"}
+  #
+  # The optional `pattern` parameter is a pattern or a string. Only input strings
+  # that match the pattern or start with the string are included in the output
+  # hash.
+  #
+  #     %w{ fast boat day }.abbrev(/^.a/)
+  #     #=> {"fast"=>"fast", "fas"=>"fast", "fa"=>"fast", "day"=>"day", "da"=>"day"}
+  #
+  #     Abbrev.abbrev(%w{car box cone}, "ca")
+  #     #=> {"car"=>"car", "ca"=>"car"}
+  #
+  # See also Abbrev.abbrev
+  #
+  def abbrev: (?String | Regexp | nil) -> Hash[String, String]
+end
diff --git a/gems/abbrev/_reviewers.yaml b/gems/abbrev/_reviewers.yaml
new file mode 100644
index 000000000..0b4d3b7e8
--- /dev/null
+++ b/gems/abbrev/_reviewers.yaml
@@ -0,0 +1,2 @@
+reviewers:
+  - ksss