diff --git a/changelog/@unreleased/pr-353.v2.yml b/changelog/@unreleased/pr-353.v2.yml
new file mode 100644
index 00000000..0a71106b
--- /dev/null
+++ b/changelog/@unreleased/pr-353.v2.yml
@@ -0,0 +1,7 @@
+type: improvement
+improvement:
+  description: >
+    codecs.JSON recognizes objects implementing Marshaler and Unmarshaler.
+    Added utility types for anonymous functions implementing JSON encoding.
+  links:
+  - https://github.com/palantir/conjure-go-runtime/pull/353
diff --git a/conjure-go-contract/codecs/json.go b/conjure-go-contract/codecs/json.go
index 998eb8da..e013a6ac 100644
--- a/conjure-go-contract/codecs/json.go
+++ b/conjure-go-contract/codecs/json.go
@@ -15,10 +15,12 @@
 package codecs
 
 import (
-	"fmt"
+	"bytes"
+	"encoding/json"
 	"io"
 
 	"github.com/palantir/pkg/safejson"
+	werror "github.com/palantir/witchcraft-go-error"
 )
 
 const (
@@ -37,14 +39,29 @@ func (codecJSON) Accept() string {
 }
 
 func (codecJSON) Decode(r io.Reader, v interface{}) error {
-	if err := safejson.Decoder(r).Decode(v); err != nil {
-		return fmt.Errorf("failed to decode JSON-encoded value: %s", err.Error())
+	switch vv := v.(type) {
+	case jsonDecoder:
+		return werror.Convert(vv.DecodeJSON(r))
+	case json.Unmarshaler:
+		data, err := io.ReadAll(r)
+		if err != nil {
+			return werror.Convert(err)
+		}
+		return werror.Convert(vv.UnmarshalJSON(data))
+	default:
+		return werror.Convert(safejson.Decoder(r).Decode(v))
 	}
-	return nil
 }
 
 func (c codecJSON) Unmarshal(data []byte, v interface{}) error {
-	return safejson.Unmarshal(data, v)
+	switch vv := v.(type) {
+	case json.Unmarshaler:
+		return werror.Convert(vv.UnmarshalJSON(data))
+	case jsonDecoder:
+		return werror.Convert(vv.DecodeJSON(bytes.NewReader(data)))
+	default:
+		return werror.Convert(safejson.Unmarshal(data, v))
+	}
 }
 
 func (codecJSON) ContentType() string {
@@ -52,12 +69,32 @@ func (codecJSON) ContentType() string {
 }
 
 func (codecJSON) Encode(w io.Writer, v interface{}) error {
-	if err := safejson.Encoder(w).Encode(v); err != nil {
-		return fmt.Errorf("failed to JSON-encode value: %s", err.Error())
+	switch vv := v.(type) {
+	case jsonEncoder:
+		return werror.Convert(vv.EncodeJSON(w))
+	case json.Marshaler:
+		out, err := vv.MarshalJSON()
+		if err != nil {
+			return werror.Convert(err)
+		}
+		_, err = w.Write(out)
+		return werror.Convert(err)
+	default:
+		return werror.Convert(safejson.Encoder(w).Encode(v))
 	}
-	return nil
 }
 
 func (c codecJSON) Marshal(v interface{}) ([]byte, error) {
-	return safejson.Marshal(v)
+	switch vv := v.(type) {
+	case json.Marshaler:
+		out, err := vv.MarshalJSON()
+		return out, werror.Convert(err)
+	case jsonEncoder:
+		data := bytes.NewBuffer(nil) // TODO: Use bytesbuffer pool
+		err := vv.EncodeJSON(data)
+		return data.Bytes(), werror.Convert(err)
+	default:
+		out, err := safejson.Marshal(v)
+		return out, werror.Convert(err)
+	}
 }
diff --git a/conjure-go-contract/codecs/json_test.go b/conjure-go-contract/codecs/json_test.go
new file mode 100644
index 00000000..e60435ce
--- /dev/null
+++ b/conjure-go-contract/codecs/json_test.go
@@ -0,0 +1,62 @@
+// Copyright (c) 2022 Palantir Technologies. All rights reserved.
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+//     http://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+package codecs_test
+
+import (
+	"bytes"
+	"strings"
+	"testing"
+
+	"github.com/palantir/conjure-go-runtime/v2/conjure-go-contract/codecs"
+	"github.com/stretchr/testify/require"
+)
+
+func TestJSON_UsesMethodsWhenImplemented(t *testing.T) {
+	t.Run("Marshal_Unmarshal", func(t *testing.T) {
+		obj := map[string]testJSONObject{}
+		err := codecs.JSON.Unmarshal([]byte(`{"key":null}`), &obj)
+		require.NoError(t, err)
+		require.Contains(t, obj, "key")
+		require.Equal(t, `null`, string(obj["key"].data))
+		data, err := codecs.JSON.Marshal(obj)
+		require.NoError(t, err)
+		require.Equal(t, `{"key":null}`, string(data))
+	})
+	t.Run("Encode_Decode", func(t *testing.T) {
+		obj := map[string]testJSONObject{}
+		r := strings.NewReader(`{"key":"abc"}`)
+		err := codecs.JSON.Decode(r, &obj)
+		require.NoError(t, err)
+		require.Contains(t, obj, "key")
+		require.Equal(t, `"abc"`, string(obj["key"].data))
+		out := bytes.Buffer{}
+		err = codecs.JSON.Encode(&out, obj)
+		require.NoError(t, err)
+		require.Equal(t, "{\"key\":\"abc\"}\n", out.String())
+	})
+}
+
+type testJSONObject struct {
+	data []byte
+}
+
+func (t testJSONObject) MarshalJSON() ([]byte, error) {
+	return t.data, nil
+}
+
+func (t *testJSONObject) UnmarshalJSON(data []byte) error {
+	t.data = data
+	return nil
+}
diff --git a/conjure-go-contract/codecs/json_types.go b/conjure-go-contract/codecs/json_types.go
new file mode 100644
index 00000000..b496e156
--- /dev/null
+++ b/conjure-go-contract/codecs/json_types.go
@@ -0,0 +1,63 @@
+// Copyright (c) 2022 Palantir Technologies. All rights reserved.
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+//     http://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+package codecs
+
+import (
+	"io"
+)
+
+// jsonDecoder is an interface which may be implemented by objects passed to the Decode and Unmarshal methods.
+// If implemented, the standard json.Decoder is bypassed.
+type jsonDecoder interface {
+	DecodeJSON(r io.Reader) error
+}
+
+// JSONDecoderFunc is an alias type which implements jsonDecoder.
+// It can be used to declare anonymous/dynamic unmarshal logic.
+type JSONDecoderFunc func(r io.Reader) error
+
+func (f JSONDecoderFunc) DecodeJSON(r io.Reader) error {
+	return f(r)
+}
+
+// JSONUnmarshalFunc is an alias type which implements json.Unmarshaler.
+// It can be used to declare anonymous/dynamic unmarshal logic.
+type JSONUnmarshalFunc func([]byte) error
+
+func (f JSONUnmarshalFunc) UnmarshalJSON(data []byte) error {
+	return f(data)
+}
+
+// jsonEncoder is an interface which may be implemented by objects passed to the Encode and Marshal methods.
+// If implemented, the standard json.Encoder is bypassed.
+type jsonEncoder interface {
+	EncodeJSON(w io.Writer) error
+}
+
+// JSONEncoderFunc is an alias type which implements jsonEncoder.
+// It can be used to declare anonymous/dynamic marshal logic.
+type JSONEncoderFunc func(w io.Writer) error
+
+func (f JSONEncoderFunc) EncodeJSON(w io.Writer) error {
+	return f(w)
+}
+
+// JSONMarshalFunc is an alias type which implements json.Marshaler.
+// It can be used to declare anonymous/dynamic marshal logic.
+type JSONMarshalFunc func() ([]byte, error)
+
+func (f JSONMarshalFunc) MarshalJSON() ([]byte, error) {
+	return f()
+}