diff --git a/lib/diver_down/web.rb b/lib/diver_down/web.rb
index 53c1ab1..db7d304 100644
--- a/lib/diver_down/web.rb
+++ b/lib/diver_down/web.rb
@@ -45,7 +45,8 @@ def call(env)
           page: request.params['page']&.to_i || 1,
           per: request.params['per']&.to_i || 100,
           title: request.params['title'] || '',
-          source: request.params['source'] || ''
+          source: request.params['source'] || '',
+          definition_group: request.params['definition_group'] || ''
         )
       in ['GET', %r{\A/api/sources\.json\z}]
         action.sources
diff --git a/lib/diver_down/web/action.rb b/lib/diver_down/web/action.rb
index 8005e5d..274913c 100644
--- a/lib/diver_down/web/action.rb
+++ b/lib/diver_down/web/action.rb
@@ -119,8 +119,9 @@ def module(module_names)
       # @param per [Integer]
       # @param title [String]
       # @param source [String]
-      def definitions(page:, per:, title:, source:)
-        definition_enumerator = DiverDown::Web::DefinitionEnumerator.new(@store, title:, source:)
+      # @param definition_group [String]
+      def definitions(page:, per:, title:, source:, definition_group:)
+        definition_enumerator = DiverDown::Web::DefinitionEnumerator.new(@store, title:, source:, definition_group:)
         definitions, pagination = paginate(definition_enumerator, page, per)
 
         json(
diff --git a/lib/diver_down/web/definition_enumerator.rb b/lib/diver_down/web/definition_enumerator.rb
index 9edbbef..d37a624 100644
--- a/lib/diver_down/web/definition_enumerator.rb
+++ b/lib/diver_down/web/definition_enumerator.rb
@@ -7,10 +7,12 @@ class DefinitionEnumerator
 
       # @param store [DiverDown::Definition::Store]
       # @param query [String]
-      def initialize(store, title: '', source: '')
+      # @param definition_group [String]
+      def initialize(store, title: '', source: '', definition_group: '')
         @store = store
         @title = title
         @source = source
+        @definition_group = definition_group
       end
 
       # @yield [parent_bit_id, bit_id, definition]
@@ -47,6 +49,10 @@ def match_definition?(definition)
           matched &&= definition.sources.any? { _1.source_name.include?(@source) }
         end
 
+        unless @definition_group.empty?
+          matched &&= definition.definition_group.to_s.include?(@definition_group)
+        end
+
         matched
       end
     end
diff --git a/spec/diver_down/web/definition_enumerator_spec.rb b/spec/diver_down/web/definition_enumerator_spec.rb
index 4fc3f63..2c97507 100644
--- a/spec/diver_down/web/definition_enumerator_spec.rb
+++ b/spec/diver_down/web/definition_enumerator_spec.rb
@@ -26,6 +26,45 @@
         )
       end
 
+      describe 'with definition_group' do
+        def assert_query(store, definition_group, expected)
+          actual = described_class.new(store, definition_group:).each.to_a
+          expect(actual).to eq(expected), -> {
+            "definition_group: #{definition_group.inspect}\n" \
+            "expected: #{expected.inspect}\n" \
+            "actual: #{actual.inspect}"
+          }
+        end
+
+        it 'filters by definition_group' do
+          store = DiverDown::Web::DefinitionStore.new
+
+          definition_1 = DiverDown::Definition.new(
+            definition_group: 'group_1',
+            sources: [
+              DiverDown::Definition::Source.new(
+                source_name: 'a.rb'
+              ),
+            ]
+          )
+          definition_2 = DiverDown::Definition.new(
+            definition_group: 'group_2',
+            sources: [
+              DiverDown::Definition::Source.new(
+                source_name: 'b.rb'
+              ),
+            ]
+          )
+
+          store.set(definition_1, definition_2)
+
+          assert_query store, 'unknown', []
+          assert_query store, 'group', [definition_1, definition_2]
+          assert_query store, 'group_1', [definition_1]
+          assert_query store, 'group_2', [definition_2]
+        end
+      end
+
       describe 'with title' do
         def assert_query(store, title, expected)
           actual = described_class.new(store, title:).each.to_a
diff --git a/spec/diver_down/web_spec.rb b/spec/diver_down/web_spec.rb
index bc15d49..37e0aa0 100644
--- a/spec/diver_down/web_spec.rb
+++ b/spec/diver_down/web_spec.rb
@@ -184,6 +184,47 @@ def assert_source(source, expected_ids)
         assert_source 'b.', [definition_2_id]
       end
     end
+
+    describe 'definition_group' do
+      def assert_definition_group(definition_group, expected_ids)
+        get "/api/definitions.json?definition_group=#{definition_group}"
+
+        definitions = JSON.parse(last_response.body)['definitions']
+        ids = definitions.map { _1['id'] }
+
+        expect(ids).to match_array(expected_ids), -> {
+          "definition_group: #{definition_group.inspect}\n" \
+          "expected_ids: #{expected_ids.inspect}\n" \
+          "actual_ids: #{ids.inspect}"
+        }
+      end
+
+      it 'filters definitions by definition_group=value' do
+        definition_1 = DiverDown::Definition.new(
+          definition_group: 'group_1',
+          sources: [
+            DiverDown::Definition::Source.new(
+              source_name: 'a.rb'
+            ),
+          ]
+        )
+        definition_2 = DiverDown::Definition.new(
+          definition_group: 'group_2',
+          sources: [
+            DiverDown::Definition::Source.new(
+              source_name: 'b.rb'
+            ),
+          ]
+        )
+
+        definition_1_id, definition_2_id = store.set(definition_1, definition_2)
+
+        assert_definition_group 'unknown', []
+        assert_definition_group 'group', [definition_1_id, definition_2_id]
+        assert_definition_group 'group_1', [definition_1_id]
+        assert_definition_group 'group_2', [definition_2_id]
+      end
+    end
   end
 
   describe 'GET /api/initialization_status.json' do