diff --git a/src/wp-includes/rest-api/endpoints/class-wp-rest-sidebars-controller.php b/src/wp-includes/rest-api/endpoints/class-wp-rest-sidebars-controller.php
index 4c1e82fbdb..3ab7dc993b 100644
--- a/src/wp-includes/rest-api/endpoints/class-wp-rest-sidebars-controller.php
+++ b/src/wp-includes/rest-api/endpoints/class-wp-rest-sidebars-controller.php
@@ -98,8 +98,10 @@ class WP_REST_Sidebars_Controller extends WP_REST_Controller {
 	 * @return WP_REST_Response|WP_Error Response object on success, or WP_Error object on failure.
 	 */
 	public function get_items( $request ) {
+		retrieve_widgets();
+
 		$data = array();
-		foreach ( (array) wp_get_sidebars_widgets() as $id => $widgets ) {
+		foreach ( wp_get_sidebars_widgets() as $id => $widgets ) {
 			$sidebar = $this->get_sidebar( $id );
 
 			if ( ! $sidebar ) {
@@ -135,6 +137,8 @@ class WP_REST_Sidebars_Controller extends WP_REST_Controller {
 	 * @return WP_REST_Response|WP_Error Response object on success, or WP_Error object on failure.
 	 */
 	public function get_item( $request ) {
+		retrieve_widgets();
+
 		$sidebar = $this->get_sidebar( $request['id'] );
 
 		if ( ! $sidebar ) {
diff --git a/tests/phpunit/tests/rest-api/rest-sidebars-controller.php b/tests/phpunit/tests/rest-api/rest-sidebars-controller.php
index ff9d8dfba5..a14b6bf17b 100644
--- a/tests/phpunit/tests/rest-api/rest-sidebars-controller.php
+++ b/tests/phpunit/tests/rest-api/rest-sidebars-controller.php
@@ -8,10 +8,11 @@
  */
 
 /**
- * Tests for REST API for Menus.
+ * Tests for REST API for Widgets.
  *
  * @see WP_Test_REST_Controller_Testcase
  * @group restapi
+ * @group widgets
  * @covers WP_REST_Sidebars_Controller
  */
 class WP_Test_REST_Sidebars_Controller extends WP_Test_REST_Controller_Testcase {
@@ -61,6 +62,18 @@ class WP_Test_REST_Sidebars_Controller extends WP_Test_REST_Controller_Testcase
 		update_option( 'sidebars_widgets', array() );
 	}
 
+	function clean_up_global_scope() {
+		global $wp_widget_factory, $wp_registered_sidebars, $wp_registered_widgets, $wp_registered_widget_controls, $wp_registered_widget_updates;
+
+		$wp_registered_sidebars        = array();
+		$wp_registered_widgets         = array();
+		$wp_registered_widget_controls = array();
+		$wp_registered_widget_updates  = array();
+		$wp_widget_factory->widgets    = array();
+
+		parent::clean_up_global_scope();
+	}
+
 	private function setup_widget( $option_name, $number, $settings ) {
 		update_option(
 			$option_name,
@@ -128,6 +141,8 @@ class WP_Test_REST_Sidebars_Controller extends WP_Test_REST_Controller_Testcase
 	 * @ticket 41683
 	 */
 	public function test_get_items() {
+		wp_widgets_init();
+
 		$request  = new WP_REST_Request( 'GET', '/wp/v2/sidebars' );
 		$response = rest_get_server()->dispatch( $request );
 		$data     = $response->get_data();
@@ -193,6 +208,8 @@ class WP_Test_REST_Sidebars_Controller extends WP_Test_REST_Controller_Testcase
 	 * @ticket 41683
 	 */
 	public function test_get_items_active_sidebar_with_widgets() {
+		wp_widgets_init();
+
 		$this->setup_widget(
 			'widget_rss',
 			1,
@@ -241,6 +258,56 @@ class WP_Test_REST_Sidebars_Controller extends WP_Test_REST_Controller_Testcase
 		);
 	}
 
+	/**
+	 * @ticket 53489
+	 */
+	public function test_get_items_when_registering_new_sidebars() {
+		register_sidebar(
+			array(
+				'name'          => 'New Sidebar',
+				'id'            => 'new-sidebar',
+				'before_widget' => '',
+				'after_widget'  => '',
+				'before_title'  => '',
+				'after_title'   => '',
+			)
+		);
+
+		$request  = new WP_REST_Request( 'GET', '/wp/v2/sidebars' );
+		$response = rest_get_server()->dispatch( $request );
+		$data     = $response->get_data();
+		$data     = $this->remove_links( $data );
+		$this->assertSame(
+			array(
+				array(
+					'id'            => 'wp_inactive_widgets',
+					'name'          => 'Inactive widgets',
+					'description'   => '',
+					'class'         => '',
+					'before_widget' => '',
+					'after_widget'  => '',
+					'before_title'  => '',
+					'after_title'   => '',
+					'status'        => 'inactive',
+					'widgets'       => array(),
+				),
+				array(
+					'id'            => 'new-sidebar',
+					'name'          => 'New Sidebar',
+					'description'   => '',
+					'class'         => '',
+					'before_widget' => '',
+					'after_widget'  => '',
+					'before_title'  => '',
+					'after_title'   => '',
+					'status'        => 'active',
+					'widgets'       => array(),
+				),
+			),
+			$data
+		);
+	}
+
 	/**
 	 * @ticket 41683
 	 */
@@ -317,6 +384,8 @@ class WP_Test_REST_Sidebars_Controller extends WP_Test_REST_Controller_Testcase
 	 * @ticket 41683
 	 */
 	public function test_update_item() {
+		wp_widgets_init();
+
 		$this->setup_widget(
 			'widget_rss',
 			1,
@@ -382,6 +451,8 @@ class WP_Test_REST_Sidebars_Controller extends WP_Test_REST_Controller_Testcase
 	 * @ticket 41683
 	 */
 	public function test_update_item_removes_widget_from_existing_sidebar() {
+		wp_widgets_init();
+
 		$this->setup_widget(
 			'widget_text',
 			1,
@@ -423,6 +494,8 @@ class WP_Test_REST_Sidebars_Controller extends WP_Test_REST_Controller_Testcase
 	 * @ticket 41683
 	 */
 	public function test_update_item_moves_omitted_widget_to_inactive_sidebar() {
+		wp_widgets_init();
+
 		$this->setup_widget(
 			'widget_text',
 			1,
@@ -465,6 +538,8 @@ class WP_Test_REST_Sidebars_Controller extends WP_Test_REST_Controller_Testcase
 	 * @ticket 41683
 	 */
 	public function test_get_items_inactive_widgets() {
+		wp_widgets_init();
+
 		$this->setup_widget(
 			'widget_rss',
 			1,
diff --git a/tests/phpunit/tests/rest-api/rest-widget-types-controller.php b/tests/phpunit/tests/rest-api/rest-widget-types-controller.php
index 6082931e42..cd84b91c48 100644
--- a/tests/phpunit/tests/rest-api/rest-widget-types-controller.php
+++ b/tests/phpunit/tests/rest-api/rest-widget-types-controller.php
@@ -112,6 +112,7 @@ class WP_Test_REST_Widget_Types_Controller extends WP_Test_REST_Controller_Testc
 	 * @ticket 41683
 	 */
 	public function test_get_items() {
+		wp_widgets_init();
 		wp_set_current_user( self::$admin_id );
 		$request  = new WP_REST_Request( 'GET', '/wp/v2/widget-types' );
 		$response = rest_get_server()->dispatch( $request );