diff --git a/lib/gitlab/middleware/read_only/controller.rb b/lib/gitlab/middleware/read_only/controller.rb
index f142f9da43d..817db12ac55 100644
--- a/lib/gitlab/middleware/read_only/controller.rb
+++ b/lib/gitlab/middleware/read_only/controller.rb
@@ -71,12 +71,16 @@ module Gitlab
           @route_hash ||= Rails.application.routes.recognize_path(request.url, { method: request.request_method }) rescue {}
         end
 
-        # Overridden in EE module
-        def whitelisted_routes
-          grack_route || ReadOnly.internal_routes.any? { |path| request.path.include?(path) } || lfs_route || sidekiq_route
+        def relative_url
+          File.join('', Gitlab.config.gitlab.relative_url_root).chomp('/')
         end
 
-        def grack_route
+        # Overridden in EE module
+        def whitelisted_routes
+          grack_route? || internal_route? || lfs_route? || sidekiq_route?
+        end
+
+        def grack_route?
           # Calling route_hash may be expensive. Only do it if we think there's a possible match
           return false unless
             request.path.end_with?('.git/git-upload-pack', '.git/git-receive-pack')
@@ -84,7 +88,11 @@ module Gitlab
           WHITELISTED_GIT_ROUTES[route_hash[:controller]]&.include?(route_hash[:action])
         end
 
-        def lfs_route
+        def internal_route?
+          ReadOnly.internal_routes.any? { |path| request.path.include?(path) }
+        end
+
+        def lfs_route?
           # Calling route_hash may be expensive. Only do it if we think there's a possible match
           unless request.path.end_with?('/info/lfs/objects/batch',
             '/info/lfs/locks', '/info/lfs/locks/verify') ||
@@ -95,8 +103,8 @@ module Gitlab
           WHITELISTED_GIT_LFS_ROUTES[route_hash[:controller]]&.include?(route_hash[:action])
         end
 
-        def sidekiq_route
-          request.path.start_with?('/admin/sidekiq')
+        def sidekiq_route?
+          request.path.start_with?("#{relative_url}/admin/sidekiq")
         end
       end
     end
diff --git a/spec/lib/gitlab/middleware/read_only_spec.rb b/spec/lib/gitlab/middleware/read_only_spec.rb
index bdb1f34d2f6..24d49a049b6 100644
--- a/spec/lib/gitlab/middleware/read_only_spec.rb
+++ b/spec/lib/gitlab/middleware/read_only_spec.rb
@@ -101,16 +101,36 @@ describe Gitlab::Middleware::ReadOnly do
         expect(subject).not_to disallow_request
       end
 
-      it 'expects requests to sidekiq admin to be allowed' do
-        response = request.post('/admin/sidekiq')
+      context 'sidekiq admin requests' do
+        where(:mounted_at) do
+          [
+            '',
+            '/',
+            '/gitlab',
+            '/gitlab/',
+            '/gitlab/gitlab',
+            '/gitlab/gitlab/'
+          ]
+        end
 
-        expect(response).not_to be_redirect
-        expect(subject).not_to disallow_request
+        with_them do
+          before do
+            stub_config_setting(relative_url_root: mounted_at)
+          end
 
-        response = request.get('/admin/sidekiq')
+          it 'allows requests' do
+            path = File.join(mounted_at, 'admin/sidekiq')
+            response = request.post(path)
 
-        expect(response).not_to be_redirect
-        expect(subject).not_to disallow_request
+            expect(response).not_to be_redirect
+            expect(subject).not_to disallow_request
+
+            response = request.get(path)
+
+            expect(response).not_to be_redirect
+            expect(subject).not_to disallow_request
+          end
+        end
       end
 
       where(:description, :path) do