diff --git a/CHANGELOG.md b/CHANGELOG.md
index 67e0171..ee4b3c4 100644
--- a/CHANGELOG.md
+++ b/CHANGELOG.md
@@ -3,7 +3,9 @@
 * support array-form `type` (3.1) in value validation
 * support root-level `jsonSchemaDialect` (OpenAPI 3.1) in the parse layer
 * support `prefixItems` (OpenAPI 3.1) with positional tuple validation
+* support `contentMediaType` (OpenAPI 3.1) in the parse layer
 * add `SpecValidator` with `strict_specification_version` config (`:silent` / `:warn` / `:raise`) to detect version mismatches between declared OpenAPI version and actual field usage
+  * `ContentMediaTypeIn30`: detect `contentMediaType` usage in 3.0 documents (3.1 addition)
   * `PrefixItemsIn30`: detect `prefixItems` usage in 3.0 documents (3.1 addition)
   * `JsonSchemaDialectIn30`: detect root-level `jsonSchemaDialect` usage in 3.0 documents (3.1 addition)
   * `TypeArrayIn30`: detect array-form `type` usage in 3.0 documents (3.1 form)
diff --git a/lib/openapi_parser/schemas/schema.rb b/lib/openapi_parser/schemas/schema.rb
index 5436a3f..2a438f2 100644
--- a/lib/openapi_parser/schemas/schema.rb
+++ b/lib/openapi_parser/schemas/schema.rb
@@ -72,7 +72,8 @@ class Schema < Base
                         :nullable,
                         :example,
                         :deprecated,
-                        :const
+                        :const,
+                        :contentMediaType
 
     # @!attribute [r] read_only
     #   @return [Boolean, nil]
diff --git a/lib/openapi_parser/spec_validator.rb b/lib/openapi_parser/spec_validator.rb
index f239dec..edc4963 100644
--- a/lib/openapi_parser/spec_validator.rb
+++ b/lib/openapi_parser/spec_validator.rb
@@ -2,6 +2,7 @@
 require_relative 'spec_validator/rule'
 require_relative 'spec_validator/rules/exclusive_minimum'
 require_relative 'spec_validator/rules/exclusive_maximum'
+require_relative 'spec_validator/rules/content_media_type_in_30'
 require_relative 'spec_validator/rules/prefix_items_in_30'
 require_relative 'spec_validator/rules/json_schema_dialect_in_30'
 require_relative 'spec_validator/rules/type_array_in_30'
@@ -61,6 +62,7 @@ def rules
         [
           Rules::ExclusiveMinimum,
           Rules::ExclusiveMaximum,
+          Rules::ContentMediaTypeIn30,
           Rules::PrefixItemsIn30,
           Rules::JsonSchemaDialectIn30,
           Rules::TypeArrayIn30,
diff --git a/lib/openapi_parser/spec_validator/rules/content_media_type_in_30.rb b/lib/openapi_parser/spec_validator/rules/content_media_type_in_30.rb
new file mode 100644
index 0000000..ba6908f
--- /dev/null
+++ b/lib/openapi_parser/spec_validator/rules/content_media_type_in_30.rb
@@ -0,0 +1,25 @@
+module OpenAPIParser
+  class SpecValidator
+    module Rules
+      # `contentMediaType` is a JSON Schema 2020-12 annotation adopted by
+      # 3.1. It is metadata, so it has no runtime side-effects; we only
+      # report version mismatches.
+      class ContentMediaTypeIn30 < Rule
+        def check(root)
+          return [] unless version == :v3_0
+
+          violations = []
+          each_schema(root) do |schema|
+            next unless schema.raw_schema.is_a?(Hash) && schema.raw_schema.key?('contentMediaType')
+
+            violations << violation(
+              path: schema.object_reference,
+              message: '`contentMediaType` is a 3.1 addition (from JSON Schema 2020-12); 3.0 has no equivalent',
+            )
+          end
+          violations
+        end
+      end
+    end
+  end
+end
diff --git a/sig/openapi_parser/spec_validator.rbs b/sig/openapi_parser/spec_validator.rbs
index c634973..da58edd 100644
--- a/sig/openapi_parser/spec_validator.rbs
+++ b/sig/openapi_parser/spec_validator.rbs
@@ -45,6 +45,10 @@ module OpenAPIParser
         def check: (OpenAPIParser::Schemas::OpenAPI root) -> Array[SpecValidator::SpecViolation]
       end
 
+      class ContentMediaTypeIn30 < Rule
+        def check: (OpenAPIParser::Schemas::OpenAPI root) -> Array[SpecValidator::SpecViolation]
+      end
+
       class PrefixItemsIn30 < Rule
         def check: (OpenAPIParser::Schemas::OpenAPI root) -> Array[SpecValidator::SpecViolation]
       end
diff --git a/spec/data/openapi_3_1/content_media_type_30.yaml b/spec/data/openapi_3_1/content_media_type_30.yaml
new file mode 100644
index 0000000..dc53d1e
--- /dev/null
+++ b/spec/data/openapi_3_1/content_media_type_30.yaml
@@ -0,0 +1,24 @@
+openapi: 3.0.3
+info:
+  title: Document Upload API
+  version: '1.0'
+paths:
+  /documents:
+    post:
+      summary: Upload a document
+      requestBody:
+        content:
+          application/json:
+            schema:
+              $ref: '#/components/schemas/Attachment'
+      responses:
+        '201':
+          description: Created
+components:
+  schemas:
+    Attachment:
+      type: string
+      # `contentMediaType` is a JSON Schema 2020-12 annotation adopted by 3.1;
+      # 3.0 has no equivalent, so its use on a 3.0 document is a spec
+      # violation.
+      contentMediaType: application/json
diff --git a/spec/data/openapi_3_1/content_media_type_31.yaml b/spec/data/openapi_3_1/content_media_type_31.yaml
new file mode 100644
index 0000000..ed683b5
--- /dev/null
+++ b/spec/data/openapi_3_1/content_media_type_31.yaml
@@ -0,0 +1,23 @@
+openapi: 3.1.0
+info:
+  title: Document Upload API
+  version: '1.0'
+paths:
+  /documents:
+    post:
+      summary: Upload a document
+      requestBody:
+        content:
+          application/json:
+            schema:
+              $ref: '#/components/schemas/Attachment'
+      responses:
+        '201':
+          description: Created
+components:
+  schemas:
+    Attachment:
+      type: string
+      # `contentMediaType` is legitimate under 3.1 (JSON Schema 2020-12), so
+      # no violation is expected here.
+      contentMediaType: application/json
diff --git a/spec/openapi_parser/spec_validator/integration_3_1_spec.rb b/spec/openapi_parser/spec_validator/integration_3_1_spec.rb
index 53fb8f4..6800dff 100644
--- a/spec/openapi_parser/spec_validator/integration_3_1_spec.rb
+++ b/spec/openapi_parser/spec_validator/integration_3_1_spec.rb
@@ -74,6 +74,20 @@ def expect_clean(file)
     end
   end
 
+  describe 'contentMediaType (JSON Schema 2020-12 annotation new in 3.1)' do
+    it 'warns on the version-mismatched document under :warn' do
+      expect_mismatch_warns('content_media_type_30.yaml', [:content_media_type_in30])
+    end
+
+    it 'raises SpecViolationError on the version-mismatched document under :raise' do
+      expect_mismatch_raises('content_media_type_30.yaml', [:content_media_type_in30])
+    end
+
+    it 'stays clean on the correctly-versioned document' do
+      expect_clean('content_media_type_31.yaml')
+    end
+  end
+
   describe 'prefixItems (JSON Schema tuple keyword new in 3.1)' do
     it 'warns on the version-mismatched document under :warn' do
       expect_mismatch_warns('prefix_items_30.yaml', [:prefix_items_in30])
diff --git a/spec/openapi_parser/spec_validator/rules/content_media_type_in_30_spec.rb b/spec/openapi_parser/spec_validator/rules/content_media_type_in_30_spec.rb
new file mode 100644
index 0000000..6f4526b
--- /dev/null
+++ b/spec/openapi_parser/spec_validator/rules/content_media_type_in_30_spec.rb
@@ -0,0 +1,64 @@
+require_relative '../../../spec_helper'
+
+RSpec.describe 'OpenAPIParser::SpecValidator::Rules::ContentMediaTypeIn30' do
+  def base_doc(openapi_version_string, sample_schema)
+    {
+      'openapi' => openapi_version_string,
+      'info' => { 'title' => 'test', 'version' => '1.0' },
+      'paths' => {},
+      'components' => { 'schemas' => { 'Sample' => sample_schema } },
+    }
+  end
+
+  def doc_with_content_media_type(openapi_version_string)
+    raw = base_doc(openapi_version_string, { 'type' => 'string', 'contentMediaType' => 'image/png' })
+    OpenAPIParser.parse(raw, strict_reference_validation: false)
+  end
+
+  def doc_without_content_media_type(openapi_version_string)
+    raw = base_doc(openapi_version_string, { 'type' => 'string' })
+    OpenAPIParser.parse(raw, strict_reference_validation: false)
+  end
+
+  def run_rule_for(root)
+    OpenAPIParser::SpecValidator::Rules::ContentMediaTypeIn30.new(root.openapi_version).check(root)
+  end
+
+  context 'with a 3.1 document using contentMediaType' do
+    it 'reports no violation' do
+      root = doc_with_content_media_type('3.1.0')
+      expect(run_rule_for(root)).to eq []
+    end
+  end
+
+  context 'with a 3.1 document without contentMediaType' do
+    it 'reports no violation' do
+      root = doc_without_content_media_type('3.1.0')
+      expect(run_rule_for(root)).to eq []
+    end
+  end
+
+  context 'with a 3.0 document using contentMediaType' do
+    it 'reports one violation pointing at the offending schema' do
+      root = doc_with_content_media_type('3.0.0')
+      violations = run_rule_for(root)
+      expect(violations.size).to eq 1
+      expect(violations.first.path).to eq '#/components/schemas/Sample'
+      expect(violations.first.rule_name).to eq :content_media_type_in30
+    end
+  end
+
+  context 'with a 3.0 document without contentMediaType' do
+    it 'reports no violation' do
+      root = doc_without_content_media_type('3.0.0')
+      expect(run_rule_for(root)).to eq []
+    end
+  end
+
+  context 'with an :unknown version document' do
+    it 'reports no violation (rule skipped)' do
+      root = doc_with_content_media_type('4.0.0')
+      expect(run_rule_for(root)).to eq []
+    end
+  end
+end