diff --git a/lib/openapi_parser/spec_validator.rb b/lib/openapi_parser/spec_validator.rb
index 8f52f37..89c6838 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/nullable_deprecation'
 
 module OpenAPIParser
   class SpecViolationError < OpenAPIError
@@ -51,6 +52,7 @@ def rules
         [
           Rules::ExclusiveMinimum,
           Rules::ExclusiveMaximum,
+          Rules::NullableDeprecation,
         ]
       end
   end
diff --git a/lib/openapi_parser/spec_validator/rules/nullable_deprecation.rb b/lib/openapi_parser/spec_validator/rules/nullable_deprecation.rb
new file mode 100644
index 0000000..29f7fb3
--- /dev/null
+++ b/lib/openapi_parser/spec_validator/rules/nullable_deprecation.rb
@@ -0,0 +1,25 @@
+module OpenAPIParser
+  class SpecValidator
+    module Rules
+      # `nullable` is a 3.0 keyword that 3.1 removed in favor of
+      # `type: [..., "null"]`. The field's mere presence on a 3.1 document
+      # is a spec violation, regardless of true/false.
+      class NullableDeprecation < Rule
+        def check(root)
+          return [] unless version == :v3_1
+
+          violations = []
+          each_schema(root) do |schema|
+            next unless schema.raw_schema.is_a?(Hash) && schema.raw_schema.key?('nullable')
+
+            violations << violation(
+              path: schema.object_reference,
+              message: '`nullable` was removed in 3.1; use `type: [..., "null"]` instead',
+            )
+          end
+          violations
+        end
+      end
+    end
+  end
+end
diff --git a/sig/openapi_parser/spec_validator.rbs b/sig/openapi_parser/spec_validator.rbs
index f680c79..fea9a7b 100644
--- a/sig/openapi_parser/spec_validator.rbs
+++ b/sig/openapi_parser/spec_validator.rbs
@@ -44,6 +44,10 @@ module OpenAPIParser
       class ExclusiveMaximum < Rule
         def check: (OpenAPIParser::Schemas::OpenAPI root) -> Array[SpecValidator::SpecViolation]
       end
+
+      class NullableDeprecation < Rule
+        def check: (OpenAPIParser::Schemas::OpenAPI root) -> Array[SpecValidator::SpecViolation]
+      end
     end
   end
 
diff --git a/spec/data/openapi_3_1/nullable_30.yaml b/spec/data/openapi_3_1/nullable_30.yaml
new file mode 100644
index 0000000..42b986a
--- /dev/null
+++ b/spec/data/openapi_3_1/nullable_30.yaml
@@ -0,0 +1,31 @@
+openapi: 3.0.3
+info:
+  title: Profile API
+  version: '1.0'
+paths:
+  /profiles/{id}:
+    get:
+      summary: Fetch a profile
+      parameters:
+        - name: id
+          in: path
+          required: true
+          schema:
+            type: string
+      responses:
+        '200':
+          description: OK
+          content:
+            application/json:
+              schema:
+                $ref: '#/components/schemas/Profile'
+components:
+  schemas:
+    Profile:
+      type: object
+      properties:
+        # 3.0 form: `nullable: true` is the legitimate way to allow null in
+        # 3.0, so no violation is expected on a 3.0 document.
+        nickname:
+          type: string
+          nullable: true
diff --git a/spec/data/openapi_3_1/nullable_31.yaml b/spec/data/openapi_3_1/nullable_31.yaml
new file mode 100644
index 0000000..afacdd8
--- /dev/null
+++ b/spec/data/openapi_3_1/nullable_31.yaml
@@ -0,0 +1,31 @@
+openapi: 3.1.0
+info:
+  title: Profile API
+  version: '1.0'
+paths:
+  /profiles/{id}:
+    get:
+      summary: Fetch a profile
+      parameters:
+        - name: id
+          in: path
+          required: true
+          schema:
+            type: string
+      responses:
+        '200':
+          description: OK
+          content:
+            application/json:
+              schema:
+                $ref: '#/components/schemas/Profile'
+components:
+  schemas:
+    Profile:
+      type: object
+      properties:
+        # 3.0 form leaking into a 3.1 document: `nullable` was removed in 3.1
+        # in favor of `type: [..., "null"]`, so its presence is a violation.
+        nickname:
+          type: string
+          nullable: true
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 3a0c4f1..143c3d3 100644
--- a/spec/openapi_parser/spec_validator/integration_3_1_spec.rb
+++ b/spec/openapi_parser/spec_validator/integration_3_1_spec.rb
@@ -73,4 +73,18 @@ def expect_clean(file)
       expect_clean('exclusive_maximum_31.yaml')
     end
   end
+
+  describe 'nullable (3.0 keyword removed in 3.1)' do
+    it 'warns on the version-mismatched document under :warn' do
+      expect_mismatch_warns('nullable_31.yaml', [:nullable_deprecation])
+    end
+
+    it 'raises SpecViolationError on the version-mismatched document under :raise' do
+      expect_mismatch_raises('nullable_31.yaml', [:nullable_deprecation])
+    end
+
+    it 'stays clean on the correctly-versioned document' do
+      expect_clean('nullable_30.yaml')
+    end
+  end
 end
diff --git a/spec/openapi_parser/spec_validator/rules/nullable_deprecation_spec.rb b/spec/openapi_parser/spec_validator/rules/nullable_deprecation_spec.rb
new file mode 100644
index 0000000..f0d5974
--- /dev/null
+++ b/spec/openapi_parser/spec_validator/rules/nullable_deprecation_spec.rb
@@ -0,0 +1,66 @@
+require_relative '../../../spec_helper'
+
+RSpec.describe 'OpenAPIParser::SpecValidator::Rules::NullableDeprecation' do
+  def schema_with_nullable(openapi_version_string, nullable_value)
+    schema_payload = { 'type' => 'string' }
+    schema_payload['nullable'] = nullable_value unless nullable_value == :absent
+    raw = {
+      'openapi' => openapi_version_string,
+      'info' => { 'title' => 'test', 'version' => '1.0' },
+      'paths' => {},
+      'components' => { 'schemas' => { 'Sample' => schema_payload } },
+    }
+    OpenAPIParser.parse(raw, strict_reference_validation: false)
+  end
+
+  def run_rule_for(root)
+    OpenAPIParser::SpecValidator::Rules::NullableDeprecation.new(root.openapi_version).check(root)
+  end
+
+  context 'with a 3.0 document using nullable: true' do
+    it 'reports no violation' do
+      root = schema_with_nullable('3.0.0', true)
+      expect(run_rule_for(root)).to eq []
+    end
+  end
+
+  context 'with a 3.0 document using nullable: false' do
+    it 'reports no violation' do
+      root = schema_with_nullable('3.0.0', false)
+      expect(run_rule_for(root)).to eq []
+    end
+  end
+
+  context 'with a 3.1 document using nullable: true' do
+    it 'reports one violation pointing at the offending schema' do
+      root = schema_with_nullable('3.1.0', true)
+      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 :nullable_deprecation
+      expect(violations.first.message).to include('removed in 3.1')
+    end
+  end
+
+  context 'with a 3.1 document using nullable: false' do
+    it 'reports one violation (the field itself is removed in 3.1)' do
+      root = schema_with_nullable('3.1.0', false)
+      violations = run_rule_for(root)
+      expect(violations.size).to eq 1
+    end
+  end
+
+  context 'with a 3.1 document that does not use nullable' do
+    it 'reports no violation' do
+      root = schema_with_nullable('3.1.0', :absent)
+      expect(run_rule_for(root)).to eq []
+    end
+  end
+
+  context 'with an :unknown version document' do
+    it 'reports no violation (rule skipped)' do
+      root = schema_with_nullable('4.0.0', true)
+      expect(run_rule_for(root)).to eq []
+    end
+  end
+end