diff --git a/app/assets/stylesheets/framework/buttons.scss b/app/assets/stylesheets/framework/buttons.scss
index ed21ad83a1c..8da8933e8a4 100644
--- a/app/assets/stylesheets/framework/buttons.scss
+++ b/app/assets/stylesheets/framework/buttons.scss
@@ -142,6 +142,10 @@
     &.btn-save {
       @include btn-outline($white-light, $green-normal, $green-normal, $green-light, $white-light, $green-light);
     }
+
+    &.btn-remove {
+      @include btn-outline($white-light, $red-normal, $red-normal, $red-light, $white-light, $red-light);
+    }
   }
 
   &.btn-gray {
diff --git a/app/controllers/projects/branches_controller.rb b/app/controllers/projects/branches_controller.rb
index 2de8ada3e29..6b9f37983c4 100644
--- a/app/controllers/projects/branches_controller.rb
+++ b/app/controllers/projects/branches_controller.rb
@@ -4,7 +4,7 @@ class Projects::BranchesController < Projects::ApplicationController
   # Authorize
   before_action :require_non_empty_project
   before_action :authorize_download_code!
-  before_action :authorize_push_code!, only: [:new, :create, :destroy]
+  before_action :authorize_push_code!, only: [:new, :create, :destroy, :destroy_all_merged]
 
   def index
     @sort = params[:sort].presence || sort_value_name
@@ -62,6 +62,13 @@ class Projects::BranchesController < Projects::ApplicationController
     end
   end
 
+  def destroy_all_merged
+    DeleteMergedBranchesService.new(@project, current_user).async_execute
+
+    redirect_to namespace_project_branches_path(@project.namespace, @project),
+      notice: 'Merged branches are being deleted. This can take some time depending on the number of branches. Please refresh the page to see changes.'
+  end
+
   private
 
   def ref
diff --git a/app/services/delete_merged_branches_service.rb b/app/services/delete_merged_branches_service.rb
new file mode 100644
index 00000000000..8b8deafedb7
--- /dev/null
+++ b/app/services/delete_merged_branches_service.rb
@@ -0,0 +1,18 @@
+require_relative 'base_service'
+
+class DeleteMergedBranchesService < BaseService
+  def async_execute
+    DeleteMergedBranchesWorker.perform_async(project.id, current_user.id)
+  end
+
+  def execute
+    raise Gitlab::Access::AccessDeniedError unless can?(current_user, :push_code, project)
+
+    branches = project.repository.branch_names
+    branches = branches.select { |branch| project.repository.merged_to_root_ref?(branch) }
+
+    branches.each do |branch|
+      DeleteBranchService.new(project, current_user).execute(branch)
+    end
+  end
+end
diff --git a/app/views/projects/branches/index.html.haml b/app/views/projects/branches/index.html.haml
index 84f38575e84..2246316b540 100644
--- a/app/views/projects/branches/index.html.haml
+++ b/app/views/projects/branches/index.html.haml
@@ -26,6 +26,8 @@
               = sort_title_oldest_updated
 
       - if can? current_user, :push_code, @project
+        = link_to namespace_project_merged_branches_path(@project.namespace, @project), class: 'btn btn-inverted btn-remove has-tooltip', title: "Delete all branches that are merged into '#{@project.repository.root_ref}'", method: :delete, data: { confirm: "Deleting the merged branches cannot be undone. Are you sure?", container: 'body' } do
+          Delete merged branches
         = link_to new_namespace_project_branch_path(@project.namespace, @project), class: 'btn btn-create' do
           New branch
 
diff --git a/app/workers/delete_merged_branches_worker.rb b/app/workers/delete_merged_branches_worker.rb
new file mode 100644
index 00000000000..f870da4ecfd
--- /dev/null
+++ b/app/workers/delete_merged_branches_worker.rb
@@ -0,0 +1,20 @@
+class DeleteMergedBranchesWorker
+  include Sidekiq::Worker
+  include DedicatedSidekiqQueue
+
+  def perform(project_id, user_id)
+    begin
+      project = Project.find(project_id)
+    rescue ActiveRecord::RecordNotFound
+      return
+    end
+
+    user = User.find(user_id)
+
+    begin
+      DeleteMergedBranchesService.new(project, user).execute
+    rescue Gitlab::Access::AccessDeniedError
+      return
+    end
+  end
+end
diff --git a/changelogs/unreleased/21076-deleted-merged-branches.yml b/changelogs/unreleased/21076-deleted-merged-branches.yml
new file mode 100644
index 00000000000..b7fa7f14384
--- /dev/null
+++ b/changelogs/unreleased/21076-deleted-merged-branches.yml
@@ -0,0 +1,4 @@
+---
+title: Add button to delete all merged branches
+merge_request: 6449
+author: Toon Claes
diff --git a/config/routes/project.rb b/config/routes/project.rb
index 8142e231621..b4ee3f68e14 100644
--- a/config/routes/project.rb
+++ b/config/routes/project.rb
@@ -300,6 +300,7 @@ resources :namespaces, path: '/', constraints: { id: /[a-zA-Z.0-9_\-]+/ }, only:
       end
 
       resources :branches, only: [:index, :new, :create, :destroy], constraints: { id: Gitlab::Regex.git_reference_regex }
+      delete :merged_branches, controller: 'branches', action: :destroy_all_merged
       resources :tags, only: [:index, :show, :new, :create, :destroy], constraints: { id: Gitlab::Regex.git_reference_regex } do
         resource :release, only: [:edit, :update]
       end
diff --git a/config/sidekiq_queues.yml b/config/sidekiq_queues.yml
index f36fe893fd0..d0e6eb93fb2 100644
--- a/config/sidekiq_queues.yml
+++ b/config/sidekiq_queues.yml
@@ -33,6 +33,7 @@
   - [project_service, 1]
   - [clear_database_cache, 1]
   - [delete_user, 1]
+  - [delete_merged_branches, 1]
   - [expire_build_instance_artifacts, 1]
   - [group_destroy, 1]
   - [irker, 1]
diff --git a/doc/api/branches.md b/doc/api/branches.md
index 0b5f7778fc7..f68eeb9f86b 100644
--- a/doc/api/branches.md
+++ b/doc/api/branches.md
@@ -240,3 +240,21 @@ Example response:
   "branch_name": "newbranch"
 }
 ```
+
+## Delete merged branches
+
+Will delete all branches that are merged into the project's default branch.
+
+```
+DELETE /projects/:id/repository/merged_branches
+```
+
+| Attribute | Type | Required | Description |
+| --------- | ---- | -------- | ----------- |
+| `id`      | integer | yes | The ID of a project |
+
+It returns `200` to indicate deletion of all merged branches was started.
+
+```bash
+curl --request DELETE --header "PRIVATE-TOKEN: 9koXpg98eAheJpvBs5tK" "https://gitlab.example.com/api/v3/projects/5/repository/merged_branches"
+```
diff --git a/lib/api/branches.rb b/lib/api/branches.rb
index 21a106387f0..73aed624ea7 100644
--- a/lib/api/branches.rb
+++ b/lib/api/branches.rb
@@ -128,6 +128,18 @@ module API
           render_api_error!(result[:message], result[:return_code])
         end
       end
+
+      # Delete all merged branches
+      #
+      # Parameters:
+      #   id (required) - The ID of a project
+      # Example Request:
+      #   DELETE /projects/:id/repository/branches/delete_merged
+      delete ":id/repository/merged_branches" do
+        DeleteMergedBranchesService.new(user_project, current_user).async_execute
+
+        status(200)
+      end
     end
   end
 end
diff --git a/spec/controllers/projects/branches_controller_spec.rb b/spec/controllers/projects/branches_controller_spec.rb
index 644de308c64..f7cf006efd6 100644
--- a/spec/controllers/projects/branches_controller_spec.rb
+++ b/spec/controllers/projects/branches_controller_spec.rb
@@ -1,13 +1,13 @@
 require 'spec_helper'
 
 describe Projects::BranchesController do
-  let(:project) { create(:project) }
-  let(:user)    { create(:user) }
+  let(:project)   { create(:project) }
+  let(:user)      { create(:user) }
+  let(:developer) { create(:user) }
 
   before do
-    sign_in(user)
-
     project.team << [user, :master]
+    project.team << [user, :developer]
 
     allow(project).to receive(:branches).and_return(['master', 'foo/bar/baz'])
     allow(project).to receive(:tags).and_return(['v1.0.0', 'v2.0.0'])
@@ -19,6 +19,8 @@ describe Projects::BranchesController do
 
     context "on creation of a new branch" do
       before do
+        sign_in(user)
+
         post :create,
           namespace_id: project.namespace.to_param,
           project_id: project.to_param,
@@ -68,6 +70,10 @@ describe Projects::BranchesController do
       let(:branch) { "1-feature-branch" }
       let!(:issue) { create(:issue, project: project) }
 
+      before do
+        sign_in(user)
+      end
+
       it 'redirects' do
         post :create,
           namespace_id: project.namespace.to_param,
@@ -94,6 +100,10 @@ describe Projects::BranchesController do
   describe "POST destroy with HTML format" do
     render_views
 
+    before do
+      sign_in(user)
+    end
+
     it 'returns 303' do
       post :destroy,
            format: :html,
@@ -109,6 +119,8 @@ describe Projects::BranchesController do
     render_views
 
     before do
+      sign_in(user)
+
       post :destroy,
            format: :js,
            id: branch,
@@ -139,4 +151,42 @@ describe Projects::BranchesController do
       it { expect(response).to have_http_status(404) }
     end
   end
+
+  describe "DELETE destroy_all_merged" do
+    def destroy_all_merged
+      delete :destroy_all_merged,
+             namespace_id: project.namespace.to_param,
+             project_id: project.to_param
+    end
+
+    context 'when user is allowed to push' do
+      before do
+        sign_in(user)
+      end
+
+      it 'redirects to branches' do
+        destroy_all_merged
+
+        expect(response).to redirect_to namespace_project_branches_path(project.namespace, project)
+      end
+
+      it 'starts worker to delete merged branches' do
+        expect_any_instance_of(DeleteMergedBranchesService).to receive(:async_execute)
+
+        destroy_all_merged
+      end
+    end
+
+    context 'when user is not allowed to push' do
+      before do
+        sign_in(developer)
+      end
+
+      it 'responds with status 404' do
+        destroy_all_merged
+
+        expect(response).to have_http_status(404)
+      end
+    end
+  end
 end
diff --git a/spec/requests/api/branches_spec.rb b/spec/requests/api/branches_spec.rb
index 1711096f4bd..8f605757186 100644
--- a/spec/requests/api/branches_spec.rb
+++ b/spec/requests/api/branches_spec.rb
@@ -299,4 +299,20 @@ describe API::API, api: true  do
       expect(json_response['message']).to eq('Cannot remove HEAD branch')
     end
   end
+
+  describe "DELETE /projects/:id/repository/merged_branches" do
+    before do
+      allow_any_instance_of(Repository).to receive(:rm_branch).and_return(true)
+    end
+
+    it 'returns 200' do
+      delete api("/projects/#{project.id}/repository/merged_branches", user)
+      expect(response).to have_http_status(200)
+    end
+
+    it 'returns a 403 error if guest' do
+      delete api("/projects/#{project.id}/repository/merged_branches", user2)
+      expect(response).to have_http_status(403)
+    end
+  end
 end
diff --git a/spec/services/delete_merged_branches_service_spec.rb b/spec/services/delete_merged_branches_service_spec.rb
new file mode 100644
index 00000000000..181488e89c7
--- /dev/null
+++ b/spec/services/delete_merged_branches_service_spec.rb
@@ -0,0 +1,54 @@
+require 'spec_helper'
+
+describe DeleteMergedBranchesService, services: true do
+  subject(:service) { described_class.new(project, project.owner) }
+
+  let(:project) { create(:project) }
+
+  context '#execute' do
+    context 'unprotected branches' do
+      before do
+        service.execute
+      end
+
+      it 'deletes a branch that was merged' do
+        expect(project.repository.branch_names).not_to include('improve/awesome')
+      end
+
+      it 'keeps branch that is unmerged' do
+        expect(project.repository.branch_names).to include('feature')
+      end
+
+      it 'keeps "master"' do
+        expect(project.repository.branch_names).to include('master')
+      end
+    end
+
+    context 'protected branches' do
+      before do
+        create(:protected_branch, name: 'improve/awesome', project: project)
+        service.execute
+      end
+
+      it 'keeps protected branch' do
+        expect(project.repository.branch_names).to include('improve/awesome')
+      end
+    end
+
+    context 'user without rights' do
+      let(:user) { create(:user) }
+
+      it 'cannot execute' do
+        expect { described_class.new(project, user).execute }.to raise_error(Gitlab::Access::AccessDeniedError)
+      end
+    end
+  end
+
+  context '#async_execute' do
+    it 'calls DeleteMergedBranchesWorker async' do
+      expect(DeleteMergedBranchesWorker).to receive(:perform_async)
+
+      service.async_execute
+    end
+  end
+end
diff --git a/spec/workers/delete_merged_branches_worker_spec.rb b/spec/workers/delete_merged_branches_worker_spec.rb
new file mode 100644
index 00000000000..d9497bd486c
--- /dev/null
+++ b/spec/workers/delete_merged_branches_worker_spec.rb
@@ -0,0 +1,19 @@
+require 'spec_helper'
+
+describe DeleteMergedBranchesWorker do
+  subject(:worker) { described_class.new }
+
+  let(:project) { create(:project) }
+
+  describe "#perform" do
+    it "calls DeleteMergedBranchesService" do
+      expect_any_instance_of(DeleteMergedBranchesService).to receive(:execute).and_return(true)
+
+      worker.perform(project.id, project.owner.id)
+    end
+
+    it "returns false when project was not found" do
+      expect(worker.perform('unknown', project.owner.id)).to be_falsy
+    end
+  end
+end