diff --git a/app/controllers/groups/milestones_controller.rb b/app/controllers/groups/milestones_controller.rb
index e52fa7660447321b35156941802c5720d288519e..6b1d418fc9a07f11e55088923883786cfe584b03 100644
--- a/app/controllers/groups/milestones_controller.rb
+++ b/app/controllers/groups/milestones_controller.rb
@@ -11,6 +11,9 @@ class Groups::MilestonesController < Groups::ApplicationController
         @milestone_states = GlobalMilestone.states_count(@projects)
         @milestones = Kaminari.paginate_array(milestones).page(params[:page])
       end
+      format.json do
+        render json: milestones.map { |m| m.for_display.slice(:title, :name) }
+      end
     end
   end
 
diff --git a/app/helpers/milestones_helper.rb b/app/helpers/milestones_helper.rb
index a230db22fa2490d61b08a70ad341a8c8ca0b479d..f346e20e8074fc93271ea4d655f5ae18802310ff 100644
--- a/app/helpers/milestones_helper.rb
+++ b/app/helpers/milestones_helper.rb
@@ -74,6 +74,8 @@ module MilestonesHelper
     project = @target_project || @project
     if project
       namespace_project_milestones_path(project.namespace, project, :json)
+    elsif @group
+      group_milestones_path(@group, :json)
     else
       dashboard_milestones_path(:json)
     end
diff --git a/changelogs/unreleased/34116-milestone-filtering-on-group-issues.yml b/changelogs/unreleased/34116-milestone-filtering-on-group-issues.yml
new file mode 100644
index 0000000000000000000000000000000000000000..8f8b5a96c2b6577e9f293265a5aa73e9b5ae8782
--- /dev/null
+++ b/changelogs/unreleased/34116-milestone-filtering-on-group-issues.yml
@@ -0,0 +1,4 @@
+---
+title: Change milestone endpoint for groups
+merge_request: 12374
+author: Takuya Noguchi
diff --git a/spec/controllers/groups/milestones_controller_spec.rb b/spec/controllers/groups/milestones_controller_spec.rb
index f3263bc177d4db75f851df34fe2e011ffda6ffaa..c6e5fb61cf9f8bfd4660414fd21f510c07196884 100644
--- a/spec/controllers/groups/milestones_controller_spec.rb
+++ b/spec/controllers/groups/milestones_controller_spec.rb
@@ -23,6 +23,21 @@ describe Groups::MilestonesController do
     project.team << [user, :master]
   end
 
+  describe "#index" do
+    it 'shows group milestones page' do
+      get :index, group_id: group.to_param
+
+      expect(response).to have_http_status(200)
+    end
+
+    it 'shows group milestones JSON' do
+      get :index, group_id: group.to_param, format: :json
+
+      expect(response).to have_http_status(200)
+      expect(response.content_type).to eq 'application/json'
+    end
+  end
+
   it_behaves_like 'milestone tabs'
 
   describe "#create" do
diff --git a/spec/helpers/milestones_helper_spec.rb b/spec/helpers/milestones_helper_spec.rb
index 3cb809d42b52f357ecca23e86336e2522cd2f213..24d4f1b49384128f10643341d90af939ae1f288b 100644
--- a/spec/helpers/milestones_helper_spec.rb
+++ b/spec/helpers/milestones_helper_spec.rb
@@ -1,6 +1,42 @@
 require 'spec_helper'
 
 describe MilestonesHelper do
+  describe '#milestones_filter_dropdown_path' do
+    let(:project) { create(:empty_project) }
+    let(:project2) { create(:empty_project) }
+    let(:group) { create(:group) }
+
+    context 'when @project present' do
+      it 'returns project milestones JSON URL' do
+        assign(:project, project)
+
+        expect(helper.milestones_filter_dropdown_path).to eq(namespace_project_milestones_path(project.namespace, project, :json))
+      end
+    end
+
+    context 'when @target_project present' do
+      it 'returns targeted project milestones JSON URL' do
+        assign(:target_project, project2)
+
+        expect(helper.milestones_filter_dropdown_path).to eq(namespace_project_milestones_path(project2.namespace, project2, :json))
+      end
+    end
+
+    context 'when @group present' do
+      it 'returns group milestones JSON URL' do
+        assign(:group, group)
+
+        expect(helper.milestones_filter_dropdown_path).to eq(group_milestones_path(group, :json))
+      end
+    end
+
+    context 'when neither of @project/@target_project/@group present' do
+      it 'returns dashboard milestones JSON URL' do
+        expect(helper.milestones_filter_dropdown_path).to eq(dashboard_milestones_path(:json))
+      end
+    end
+  end
+
   describe "#milestone_date_range" do
     def result_for(*args)
       milestone_date_range(build(:milestone, *args))