diff --git a/.rubocop_todo.yml b/.rubocop_todo.yml
index d70c632..a3b324f 100644
--- a/.rubocop_todo.yml
+++ b/.rubocop_todo.yml
@@ -1,12 +1,12 @@
 # This configuration was generated by
 # `rubocop --auto-gen-config`
-# on 2025-05-28 16:52:10 UTC using RuboCop version 1.75.8.
+# on 2026-06-10 02:07:47 UTC using RuboCop version 1.86.1.
 # The point is for the user to remove these configuration records
 # one by one as the offenses are removed from the code base.
 # Note that changes in the inspected code, or installation of new
 # versions of RuboCop, may require this file to be generated again.
 
-# Offense count: 8
+# Offense count: 10
 # Configuration parameters: AllowedMethods.
 # AllowedMethods: enums
 Lint/ConstantDefinitionInBlock:
@@ -24,14 +24,16 @@ Lint/EmptyBlock:
     - 'spec/grape-swagger/entity/attribute_parser_spec.rb'
 
 # Offense count: 1
+# Configuration parameters: AllowComments.
 Lint/EmptyClass:
   Exclude:
     - 'spec/support/shared_contexts/custom_type_parser.rb'
 
 # Offense count: 2
 # This cop supports unsafe autocorrection (--autocorrect-all).
-# Configuration parameters: AllowedMethods.
+# Configuration parameters: AllowedMethods, InferNonNilReceiver, AdditionalNilMethods.
 # AllowedMethods: instance_of?, kind_of?, is_a?, eql?, respond_to?, equal?
+# AdditionalNilMethods: present?, blank?, try, try!
 Lint/RedundantSafeNavigation:
   Exclude:
     - 'lib/grape-swagger/entity/attribute_parser.rb'
@@ -39,27 +41,27 @@ Lint/RedundantSafeNavigation:
 # Offense count: 4
 # Configuration parameters: AllowedMethods, AllowedPatterns, CountRepeatedAttributes.
 Metrics/AbcSize:
-  Max: 34
+  Max: 25
 
 # Offense count: 2
 # Configuration parameters: CountComments, CountAsOne.
 Metrics/ClassLength:
-  Max: 145
+  Max: 142
 
-# Offense count: 2
+# Offense count: 1
 # Configuration parameters: AllowedMethods, AllowedPatterns.
 Metrics/CyclomaticComplexity:
-  Max: 11
+  Max: 9
 
 # Offense count: 7
 # Configuration parameters: CountComments, CountAsOne, AllowedMethods, AllowedPatterns.
 Metrics/MethodLength:
   Max: 28
 
-# Offense count: 2
+# Offense count: 1
 # Configuration parameters: AllowedMethods, AllowedPatterns.
 Metrics/PerceivedComplexity:
-  Max: 13
+  Max: 9
 
 # Offense count: 5
 # Configuration parameters: EnforcedStyle, CheckMethodNames, CheckSymbols, AllowedIdentifiers, AllowedPatterns.
@@ -86,28 +88,31 @@ RSpec/ContextWording:
     - 'spec/support/shared_contexts/inheritance_api.rb'
     - 'spec/support/shared_contexts/this_api.rb'
 
-# Offense count: 3
+# Offense count: 5
 # Configuration parameters: IgnoredMetadata.
 RSpec/DescribeClass:
   Exclude:
     - '**/spec/features/**/*'
-    - '**/spec/issues/**/*'
     - '**/spec/requests/**/*'
     - '**/spec/routing/**/*'
     - '**/spec/system/**/*'
     - '**/spec/views/**/*'
+    - 'spec/grape-swagger/entities/additional_properties_spec.rb'
     - 'spec/grape-swagger/entities/response_model_spec.rb'
+    - 'spec/issues/44_desc_in_entity_type_spec.rb'
+    - 'spec/issues/962_polymorphic_entity_with_custom_documentation_spec.rb'
 
-# Offense count: 4
+# Offense count: 6
 # Configuration parameters: CountAsOne.
 RSpec/ExampleLength:
-  Max: 225
+  Max: 221
 
-# Offense count: 30
+# Offense count: 33
 RSpec/LeakyConstantDeclaration:
   Exclude:
     - 'spec/grape-swagger/entities/response_model_spec.rb'
-    - '**/spec/issues/**/*'
+    - 'spec/issues/44_desc_in_entity_type_spec.rb'
+    - 'spec/issues/962_polymorphic_entity_with_custom_documentation_spec.rb'
     - 'spec/support/shared_contexts/inheritance_api.rb'
     - 'spec/support/shared_contexts/this_api.rb'
 
@@ -116,7 +121,7 @@ RSpec/MultipleDescribes:
   Exclude:
     - 'spec/grape-swagger/entities/response_model_spec.rb'
 
-# Offense count: 5
+# Offense count: 6
 RSpec/MultipleExpectations:
   Max: 11
 
@@ -127,14 +132,13 @@ RSpec/NamedSubject:
   Exclude:
     - 'spec/grape-swagger/entities/response_model_spec.rb'
 
-# Offense count: 46
+# Offense count: 57
 # Configuration parameters: AllowedGroups.
 RSpec/NestedGroups:
   Max: 5
 
 # Offense count: 3
-# Configuration parameters: Include, CustomTransform, IgnoreMethods, IgnoreMetadata.
-# Include: **/*_spec.rb
+# Configuration parameters: CustomTransform, IgnoreMethods, IgnoreMetadata.
 RSpec/SpecFilePathFormat:
   Exclude:
     - '**/spec/routing/**/*'
@@ -157,3 +161,9 @@ Style/OpenStructUse:
   Exclude:
     - 'spec/grape-swagger/entities/response_model_spec.rb'
     - 'spec/support/shared_contexts/this_api.rb'
+
+# Offense count: 1
+# This cop supports unsafe autocorrection (--autocorrect-all).
+Style/ReduceToHash:
+  Exclude:
+    - 'lib/grape-swagger/entity/parser.rb'
diff --git a/CHANGELOG.md b/CHANGELOG.md
index daaf4a0..537e0c4 100644
--- a/CHANGELOG.md
+++ b/CHANGELOG.md
@@ -3,6 +3,7 @@
 #### Features
 
 * Your contribution here.
+* [#92](https://github.com/ruby-grape/grape-swagger-entity/pull/92) Add support for `additional_properties` in entity documentation, mirroring the [grape-swagger feature](https://github.com/ruby-grape/grape-swagger#additional-properties) for request parameters - [@olivier-thatch](https://github.com/olivier-thatch).
 
 #### Fixes
 
diff --git a/docs/documentation-options.md b/docs/documentation-options.md
index 85e2a87..43427f6 100644
--- a/docs/documentation-options.md
+++ b/docs/documentation-options.md
@@ -8,6 +8,7 @@ The `documentation` hash in entity exposures supports the following options:
 |--------|-------------|---------|
 | `type` | OpenAPI data type | `String`, `Integer`, `Boolean`, `Float`, `Date`, `DateTime` |
 | `is_array` | Marks field as array type | `true` / `false` |
+| `additional_properties` | Schema for free-form object values (maps) | `true`, `String`, `'string'`, `SomeEntity` |
 
 ## Description Options
 
@@ -87,3 +88,21 @@ expose :tags, documentation: {
   desc: 'Associated tags'
 }
 ```
+
+### Additional properties (maps)
+
+For object-typed fields whose values follow a single schema (e.g. a string-to-string map),
+use `additional_properties` to set the OpenAPI [`additionalProperties`](https://swagger.io/specification/v2/#model-with-mapdictionary-properties)
+key on the resulting schema.
+
+```ruby
+# Allow any additional properties
+expose :metadata, documentation: { type: Hash, additional_properties: true }
+
+# Allow any additional properties of a particular type (Ruby class or type name)
+expose :counts,   documentation: { type: Hash, additional_properties: Integer }
+expose :tags,     documentation: { type: 'object', additional_properties: 'string' }
+
+# Allow any additional properties matching a defined entity
+expose :widgets,  documentation: { type: Hash, additional_properties: WidgetEntity }
+```
diff --git a/lib/grape-swagger/entity/attribute_parser.rb b/lib/grape-swagger/entity/attribute_parser.rb
index 9067cc6..3fa6637 100644
--- a/lib/grape-swagger/entity/attribute_parser.rb
+++ b/lib/grape-swagger/entity/attribute_parser.rb
@@ -110,9 +110,29 @@ def document_data_type(documentation, data_type)
         values = values.call if values.is_a?(Proc)
         type[:enum] = values if values.is_a?(Array)
 
+        if documentation.key?(:additional_properties)
+          type[:additionalProperties] = parse_additional_properties(documentation[:additional_properties])
+        end
+
         type
       end
 
+      def parse_additional_properties(value)
+        case value
+        when String
+          { type: value.to_s }
+        when Class
+          if direct_model_type?(value) || ambiguous_model_type?(value)
+            name = GrapeSwagger::Entity::Helper.model_name(value, endpoint)
+            { '$ref' => "#/definitions/#{name}" }
+          else
+            { type: GrapeSwagger::DocMethods::DataType.call(value) }
+          end
+        else
+          value
+        end
+      end
+
       def entity_model_type(name, entity_options)
         documentation = entity_options[:documentation]
 
diff --git a/spec/grape-swagger/entities/additional_properties_spec.rb b/spec/grape-swagger/entities/additional_properties_spec.rb
new file mode 100644
index 0000000..ba542d9
--- /dev/null
+++ b/spec/grape-swagger/entities/additional_properties_spec.rb
@@ -0,0 +1,73 @@
+# frozen_string_literal: true
+
+module AdditionalPropertiesApi
+  class ValueEntity < Grape::Entity
+    expose :name, documentation: { type: 'string' }
+  end
+
+  class WithAdditionalProperties < Grape::Entity
+    expose :open_map,
+           documentation: { type: Hash, additional_properties: true }
+    expose :string_map,
+           documentation: { type: Hash, additional_properties: String }
+    expose :string_map_via_type_name,
+           documentation: { type: 'object', additional_properties: 'string' }
+    expose :entity_map,
+           documentation: { type: Hash, additional_properties: ValueEntity }
+  end
+end
+
+describe 'additional_properties' do
+  subject(:swagger_doc) do
+    get '/swagger_doc'
+    JSON.parse(last_response.body)
+  end
+
+  let(:app) do
+    Class.new(Grape::API) do
+      namespace :additional_properties do
+        desc 'Get a thing', success: AdditionalPropertiesApi::WithAdditionalProperties
+        get '/thing' do
+          present({}, with: AdditionalPropertiesApi::WithAdditionalProperties)
+        end
+      end
+
+      add_swagger_documentation format: :json
+    end
+  end
+
+  let(:properties) { swagger_doc['definitions']['AdditionalPropertiesApi_WithAdditionalProperties']['properties'] }
+
+  it 'documents boolean additional_properties' do
+    expect(properties['open_map']).to eq('type' => 'object', 'additionalProperties' => true)
+  end
+
+  it 'documents a primitive Ruby class as a typed schema' do
+    expect(properties['string_map']).to eq(
+      'type' => 'object',
+      'additionalProperties' => { 'type' => 'string' }
+    )
+  end
+
+  it 'documents a string type name as a typed schema' do
+    expect(properties['string_map_via_type_name']).to eq(
+      'type' => 'object',
+      'additionalProperties' => { 'type' => 'string' }
+    )
+  end
+
+  it 'documents an entity class as a $ref' do
+    expect(properties['entity_map']).to eq(
+      'type' => 'object',
+      'additionalProperties' => { '$ref' => '#/definitions/AdditionalPropertiesApi_ValueEntity' }
+    )
+  end
+
+  it 'registers the referenced entity in definitions' do
+    expect(swagger_doc['definitions']).to include('AdditionalPropertiesApi_ValueEntity')
+    expect(swagger_doc['definitions']['AdditionalPropertiesApi_ValueEntity']).to include(
+      'type' => 'object',
+      'properties' => { 'name' => { 'type' => 'string' } }
+    )
+  end
+end
diff --git a/spec/grape-swagger/entity/attribute_parser_spec.rb b/spec/grape-swagger/entity/attribute_parser_spec.rb
index 12ab99a..5d96196 100644
--- a/spec/grape-swagger/entity/attribute_parser_spec.rb
+++ b/spec/grape-swagger/entity/attribute_parser_spec.rb
@@ -341,6 +341,47 @@ def self.to_s
           it { is_expected.to include(type: 'object', example: example_value, default: example_value) }
         end
       end
+
+      context 'when additional_properties is set' do
+        context 'when the value is true' do
+          let(:entity_options) { { documentation: { type: Hash, additional_properties: true } } }
+
+          it { is_expected.to include(type: 'object', additionalProperties: true) }
+        end
+
+        context 'when the value is false' do
+          let(:entity_options) { { documentation: { type: Hash, additional_properties: false } } }
+
+          it { is_expected.to include(type: 'object', additionalProperties: false) }
+        end
+
+        context 'when the value is a primitive class' do
+          let(:entity_options) { { documentation: { type: Hash, additional_properties: String } } }
+
+          it { is_expected.to include(type: 'object', additionalProperties: { type: 'string' }) }
+        end
+
+        context 'when the value is a type name string' do
+          let(:entity_options) { { documentation: { type: 'object', additional_properties: 'string' } } }
+
+          it { is_expected.to include(type: 'object', additionalProperties: { type: 'string' }) }
+        end
+
+        context 'when the value is an entity class' do
+          let(:entity_options) { { documentation: { type: Hash, additional_properties: ThisApi::Entities::Tag } } }
+
+          it { is_expected.to include(type: 'object', additionalProperties: { '$ref' => '#/definitions/Tag' }) }
+        end
+
+        context 'when combined with is_array: true' do
+          let(:entity_options) do
+            { documentation: { type: 'object', is_array: true, additional_properties: 'string' } }
+          end
+
+          it { is_expected.to include(type: :array) }
+          it { is_expected.to include(items: { type: 'object', additionalProperties: { type: 'string' } }) }
+        end
+      end
     end
   end
 end