diff --git a/src/AmpWpPlugin.php b/src/AmpWpPlugin.php
index 5aa189ab7b6..0a466dee680 100644
--- a/src/AmpWpPlugin.php
+++ b/src/AmpWpPlugin.php
@@ -88,6 +88,7 @@ final class AmpWpPlugin extends ServiceBasedPlugin {
 		'url_validation_cron'               => URLValidationCron::class,
 		'save_post_validation_event'        => SavePostValidationEvent::class,
 		'background_task_deactivator'       => BackgroundTaskDeactivator::class,
+		'theme_entities_rest_controller'    => Validation\ThemeEntitiesRESTController::class,
 	];
 
 	/**
diff --git a/src/Validation/ThemeEntitiesRESTController.php b/src/Validation/ThemeEntitiesRESTController.php
new file mode 100644
index 00000000000..b0154798800
--- /dev/null
+++ b/src/Validation/ThemeEntitiesRESTController.php
@@ -0,0 +1,258 @@
+<?php
+/**
+ * REST endpoint providing post types, taxonomies, blocks, and widgets registered by the active theme.
+ *
+ * @package AMP
+ * @since 2.2
+ */
+
+namespace AmpProject\AmpWP\Validation;
+
+use AmpProject\AmpWP\DevTools\UserAccess;
+use AmpProject\AmpWP\Infrastructure\Conditional;
+use AmpProject\AmpWP\Infrastructure\Delayed;
+use AmpProject\AmpWP\Infrastructure\Registerable;
+use AmpProject\AmpWP\Infrastructure\Service;
+use WP_Error;
+use WP_REST_Controller;
+use WP_REST_Request;
+use WP_REST_Response;
+use WP_REST_Server;
+use WP_Widget_Factory;
+
+/**
+ * ThemeEntitiesRESTController class.
+ *
+ * @since 2.2
+ * @internal
+ */
+final class ThemeEntitiesRESTController extends WP_REST_Controller implements Conditional, Delayed, Service, Registerable {
+
+	/**
+	 * The path for the endpoint.
+	 * 
+	 * @var string
+	 */
+	const REST_BASE = 'theme-entities';
+
+	/**
+	 * Context used to get entities with the theme disabled.
+	 * 
+	 * @var string
+	 */
+	const CONTEXT_THEME_DISABLED = 'theme-disabled';
+
+	/**
+	 * Context used to get entities registered by the theme.
+	 * 
+	 * @var string
+	 */
+	const CONTEXT_THEME_ONLY = 'theme-only';
+
+	/**
+	 * Response schema.
+	 *
+	 * @var array
+	 */
+	protected $schema;
+
+	/**
+	 * UserAccess instance.
+	 *
+	 * @var UserAccess
+	 */
+	private $dev_tools_user_access;
+
+	/**
+	 * Returns whether the class should be created.
+	 *
+	 * @return boolean
+	 */
+	public static function is_needed() {
+		// Don't instantiate the class if the server path is not that of the REST endpoint.
+		$path = wp_parse_url( $_SERVER['REQUEST_URI'], PHP_URL_PATH ); // phpcs:ignore WordPress.Security.ValidatedSanitizedInput.InputNotValidated, WordPress.Security.ValidatedSanitizedInput.InputNotSanitized
+		return '/wp-json/amp/v1/' . self::REST_BASE === $path;
+	}
+
+	/**
+	 * Get the action to use for registering the service.
+	 *
+	 * @return string Registration action to use.
+	 */
+	public static function get_registration_action() {
+		return 'setup_theme';
+	}
+
+	/**
+	 * Constructor.
+	 * 
+	 * @param UserAccess $dev_tools_user_access UserAccess instance.
+	 */
+	public function __construct( UserAccess $dev_tools_user_access ) {
+		$this->namespace             = 'amp/v1';
+		$this->rest_base             = self::REST_BASE;
+		$this->dev_tools_user_access = $dev_tools_user_access;
+	}
+
+	/**
+	 * Sets up the controller.
+	 */
+	public function register() {
+		if ( isset( $_GET['context'] ) && self::CONTEXT_THEME_DISABLED === $_GET['context'] ) { // phpcs:ignore WordPress.Security.NonceVerification.Recommended, WordPress.Security.ValidatedSanitizedInput.InputNotValidated
+			$hooks = [
+				'pre_option_template',
+				'option_template',
+				'pre_option_stylesheet',
+				'option_stylesheet',
+			];
+
+			foreach ( $hooks as $hook ) {
+				add_filter( $hook, '__return_empty_string', 999 );
+			}
+		}
+
+		add_action( 'rest_api_init', [ $this, 'register_routes' ] );
+	}
+
+	/**
+	 * Registers all routes for the controller.
+	 */
+	public function register_routes() {
+		register_rest_route(
+			$this->namespace,
+			'/' . $this->rest_base,
+			[
+				[
+					'methods'             => WP_REST_Server::READABLE,
+					'callback'            => [ $this, 'get_results' ],
+					'args'                => [
+						'context' => [
+							'default'     => self::CONTEXT_THEME_ONLY,
+							'description' => __( 'The request context.', 'amp' ),
+							'enum'        => [
+								self::CONTEXT_THEME_DISABLED,
+								self::CONTEXT_THEME_ONLY,
+							],
+							'type'        => 'string',
+						],
+					],
+					'permission_callback' => [ $this, 'get_items_permissions_check' ],
+				],
+				'schema' => $this->get_public_item_schema(),
+			]
+		);
+	}
+
+	/**
+	 * Checks whether the current user has permission to receive URLs.
+	 *
+	 * @param  WP_REST_Request $request Full details about the request.
+	 * @return true|WP_Error True if the request has permission; WP_Error object otherwise.
+	 */
+	public function get_items_permissions_check( $request ) { // phpcs:ignore VariableAnalysis.CodeAnalysis.VariableAnalysis.UnusedVariable
+		if ( ! $this->dev_tools_user_access->is_user_enabled() ) {
+			return new WP_Error(
+				'amp_rest_no_dev_tools',
+				__( 'Sorry, you do not have access to dev tools for the AMP plugin for WordPress.', 'amp' ),
+				[ 'status' => rest_authorization_required_code() ]
+			);
+		}
+
+		return true;
+	}
+
+	/**
+	 * Provides registered blocks, post types, taxonomies, and widgets.
+	 *
+	 * @return array
+	 */
+	private function get_entities() {
+		global $wp_widget_factory;
+
+		return [
+			'blocks'     => function_exists( 'get_dynamic_block_names' ) ? get_dynamic_block_names() : [],
+			'post_types' => array_values( get_post_types() ),
+			'taxonomies' => array_values( get_taxonomies() ),
+			'widgets'    => is_a( $wp_widget_factory, WP_Widget_Factory::class ) ? array_keys( $wp_widget_factory->widgets ) : [],
+		];
+	}
+
+	/**
+	 * Retrieves compatibility results.
+	 *
+	 * @param WP_REST_Request $request Full details about the request.
+	 * @return WP_REST_Response|WP_Error Response object on success, or WP_Error object on failure.
+	 */
+	public function get_results( $request ) {
+		// As this request can be slow, we cache the result for the current theme and version.
+		$theme         = wp_get_theme();
+		$cache_key     = md5( 'amp-theme-entities' . $theme->get( 'Name' ) . $theme->get( 'Version' ) );
+		$cached_result = get_transient( $cache_key );
+	
+		if ( $cached_result ) {
+			return rest_ensure_response( $cached_result );
+		}
+
+		// If the current request is for the theme-disabled context, the filters to disable the theme will have been added in ::register.
+		if ( self::CONTEXT_THEME_DISABLED === $request['context'] ) {
+			return rest_ensure_response( $this->get_entities() );
+		}
+
+		$entities_with_theme = $this->get_entities();
+
+		// Make a request to this endpoint with the theme disabled context.
+		$disabled_theme_request = wp_remote_get(
+			add_query_arg(
+				[ 'context' => self::CONTEXT_THEME_DISABLED ],
+				str_replace( 'https', 'http', rest_url( $this->namespace . '/' . $this->rest_base ) )
+			)
+		);
+
+		$entities_without_theme = json_decode( wp_remote_retrieve_body( $disabled_theme_request ), true );
+
+		// Collect only those entities that show up only when the theme is active.
+		$theme_entities = [];
+		foreach ( array_keys( $entities_with_theme ) as $key ) {
+			$theme_entities[ $key ] = array_values( array_diff( $entities_with_theme[ $key ], $entities_without_theme[ $key ] ) );
+		}
+
+		set_transient( $cache_key, $theme_entities, 30 * DAY_IN_SECONDS );
+
+		return rest_ensure_response( $theme_entities );
+	}
+
+	/**
+	 * Retrieves the schema for plugin options provided by the endpoint.
+	 *
+	 * @return array Item schema data.
+	 */
+	public function get_item_schema() {
+		if ( ! $this->schema ) {
+			$this->schema = [
+				'$schema'    => 'http://json-schema.org/draft-04/schema#',
+				'title'      => 'amp-wp-theme-entities',
+				'type'       => 'object',
+				'properties' => [
+					'blocks'     => [
+						'items' => 'string',
+						'type'  => 'array',
+					],
+					'post_types' => [
+						'items' => 'string',
+						'type'  => 'array',
+					],
+					'taxonomies' => [
+						'items' => 'string',
+						'type'  => 'array',
+					],
+					'widgets'    => [
+						'items' => 'string',
+						'type'  => 'array',
+					],
+				],
+			];
+		}
+
+		return $this->schema;
+	}
+}
diff --git a/tests/php/src/Validation/ThemeEntitiesRESTControllerTest.php b/tests/php/src/Validation/ThemeEntitiesRESTControllerTest.php
new file mode 100644
index 00000000000..e79714cce1d
--- /dev/null
+++ b/tests/php/src/Validation/ThemeEntitiesRESTControllerTest.php
@@ -0,0 +1,237 @@
+<?php
+/**
+ * Tests for OptionsRESTController.
+ *
+ * @package AMP
+ */
+
+namespace AmpProject\AmpWP\Tests;
+
+use AmpProject\AmpWP\DevTools\UserAccess;
+use AmpProject\AmpWP\Infrastructure\Conditional;
+use AmpProject\AmpWP\Infrastructure\Delayed;
+use AmpProject\AmpWP\Infrastructure\Registerable;
+use AmpProject\AmpWP\Infrastructure\Service;
+use AmpProject\AmpWP\Tests\Helpers\PrivateAccess;
+use AmpProject\AmpWP\Validation\ThemeEntitiesRESTController;
+use WP_REST_Controller;
+use WP_REST_Request;
+use WP_UnitTestCase;
+
+/**
+ * Tests for ThemeEntitiesRESTController.
+ *
+ * @coversDefaultClass \AmpProject\AmpWP\VALIDATION\ThemeEntitiesRESTController
+ */
+class ThemeEntitiesRESTControllerTest extends WP_UnitTestCase {
+
+	use PrivateAccess;
+
+	/**
+	 * Test instance.
+	 *
+	 * @var ThemeEntitiesRESTController
+	 */
+	private $instance;
+
+	/**
+	 * UserAccess instance.
+	 *
+	 * @var UserAccess
+	 */
+	private $dev_tools_user_access;
+
+	/**
+	 * Set up.
+	 */
+	public function setUp() {
+		parent::setUp();
+
+		$this->dev_tools_user_access = new UserAccess();
+		$this->instance              = new ThemeEntitiesRESTController( $this->dev_tools_user_access );
+
+		add_filter( 'pre_http_request', [ $this, 'filter_remote_request' ], 10, 3 );
+		do_action( 'rest_api_init' );
+	}
+
+	public function tearDown() {
+		parent::tearDown();
+
+		$theme     = wp_get_theme();
+		$cache_key = md5( 'amp-theme-entities' . $theme->get( 'Name' ) . $theme->get( 'Version' ) );
+		delete_transient( $cache_key );
+	}
+
+	public function filter_remote_request( $preempt, $parsed_args, $url ) {
+		if ( false === strpos( $url, 'theme-entities&context=theme-disabled' ) ) {
+			return $preempt;
+		}
+
+		return [
+			'body' => wp_json_encode( $this->call_private_method( $this->instance, 'get_entities' ) ),
+		];
+
+	}
+
+	/** @covers __construct() */
+	public function test__construct() {
+		$this->assertInstanceOf( ThemeEntitiesRESTController::class, $this->instance );
+		$this->assertInstanceOf( WP_REST_Controller::class, $this->instance );
+		$this->assertInstanceOf( Conditional::class, $this->instance );
+		$this->assertInstanceOf( Delayed::class, $this->instance );
+		$this->assertInstanceOf( Service::class, $this->instance );
+		$this->assertInstanceOf( Registerable::class, $this->instance );
+	}
+
+	/** @covers ::register() */
+	public function test_register_with_default_context() {
+		$this->instance->register();
+
+		$this->assertEquals( 10, has_action( 'rest_api_init', [ $this->instance, 'register_routes' ] ) );
+
+		$this->assertFalse( has_filter( 'pre_option_template', '__return_empty_string' ) );
+		$this->assertFalse( has_filter( 'option_template', '__return_empty_string' ) );
+		$this->assertFalse( has_filter( 'pre_option_stylesheet', '__return_empty_string' ) );
+		$this->assertFalse( has_filter( 'option_stylesheet', '__return_empty_string' ) );
+	}
+
+	/** @covers ::register() */
+	public function test_register_with_theme_disabled_context() {
+		$_GET['context'] = ThemeEntitiesRESTController::CONTEXT_THEME_DISABLED;
+		$this->instance->register();
+
+		$this->assertEquals( 10, has_action( 'rest_api_init', [ $this->instance, 'register_routes' ] ) );
+
+		$this->assertEquals( 999, has_filter( 'pre_option_template', '__return_empty_string' ) );
+		$this->assertEquals( 999, has_filter( 'option_template', '__return_empty_string' ) );
+		$this->assertEquals( 999, has_filter( 'pre_option_stylesheet', '__return_empty_string' ) );
+		$this->assertEquals( 999, has_filter( 'option_stylesheet', '__return_empty_string' ) );
+		unset( $_GET['context'] ); // phpcs:ignore
+	}
+
+	/** @covers ::register_routes */
+	public function test_register_routes() {
+		$this->instance->register_routes();
+
+		$this->assertContains( 'amp/v1', rest_get_server()->get_namespaces() );
+		$this->assertContains( '/amp/v1/theme-entities', array_keys( rest_get_server()->get_routes( 'amp/v1' ) ) );
+	}
+
+	/**
+	 * @covers ::get_items_permissions_check()
+	 */
+	public function test_get_items_permissions_check() {
+		$this->assertWPError( $this->instance->get_items_permissions_check( new WP_REST_Request( 'GET', '/amp/v1/' . ThemeEntitiesRESTController::REST_BASE ) ) );
+
+		wp_set_current_user( $this->factory()->user->create( [ 'role' => 'administrator' ] ) );
+		$this->assertWPError( $this->instance->get_items_permissions_check( new WP_REST_Request( 'GET', '/amp/v1/' . ThemeEntitiesRESTController::REST_BASE ) ) );
+
+		$this->dev_tools_user_access->set_user_enabled( wp_get_current_user(), true );
+
+		$this->assertTrue( $this->instance->get_items_permissions_check( new WP_REST_Request( 'GET', '/amp/v1/' . ThemeEntitiesRESTController::REST_BASE ) ) );
+	}
+
+	/**
+	 * @covers ::get_results()
+	 * @covers ::get_entities()
+	 */
+	public function test_get_results_with_theme_disabled_context() {
+		$request = new WP_REST_Request( 'GET', 'amp/v1/theme-entitites' );
+		$request->set_url_params( [ 'context' => ThemeEntitiesRESTController::CONTEXT_THEME_DISABLED ] );
+
+		wp_widgets_init();
+		$data = $this->instance->get_results( $request )->get_data();
+
+		$this->assertEquals(
+			array_keys( $data ),
+			[
+				'blocks',
+				'post_types',
+				'taxonomies',
+				'widgets',
+			]
+		);
+
+		if ( function_exists( 'get_dynamic_block_names' ) ) {
+			$this->assertNotEmpty( $data['blocks'] );
+		} else {
+			$this->assertEmpty( $data['blocks'] );
+		}
+		
+		$this->assertNotEmpty( $data['post_types'] );
+		$this->assertNotEmpty( $data['taxonomies'] );
+		$this->assertNotEmpty( $data['widgets'] );
+	}
+
+	/**
+	 * @covers ::get_results()
+	 * @covers ::get_entities()
+	 */
+	public function test_get_results_with_default_context() {
+		$request = new WP_REST_Request( 'GET', 'amp/v1/theme-entitites' );
+
+		$data = $this->instance->get_results( $request )->get_data();
+
+		$this->assertEquals(
+			array_keys( $data ),
+			[
+				'blocks',
+				'post_types',
+				'taxonomies',
+				'widgets',
+			]
+		);
+
+		$this->assertEmpty( $data['blocks'] );
+		$this->assertEmpty( $data['post_types'] );
+		$this->assertEmpty( $data['taxonomies'] );
+		$this->assertEmpty( $data['widgets'] );
+	}
+
+	/**
+	 * @covers ::get_results()
+	 * @covers ::get_entities()
+	 */
+	public function test_get_results_cache() {
+		$request = new WP_REST_Request( 'GET', 'amp/v1/theme-entitites' );
+		$this->instance->get_results( $request );
+
+		$theme     = wp_get_theme();
+		$cache_key = md5( 'amp-theme-entities' . $theme->get( 'Name' ) . $theme->get( 'Version' ) );
+
+		$this->assertEquals(
+			array_keys( get_transient( $cache_key ) ),
+			[
+				'blocks',
+				'post_types',
+				'taxonomies',
+				'widgets',
+			]
+		);
+	}
+
+	/** @covers ::get_item_schema() */
+	public function test_get_item_schema() {
+		$schema = $this->instance->get_item_schema();
+
+		$this->assertEquals(
+			array_keys( $schema ),
+			[
+				'$schema',
+				'title',
+				'type',
+				'properties',
+			]
+		);
+
+		$this->assertEquals(
+			array_keys( $schema['properties'] ),
+			[
+				'blocks',
+				'post_types',
+				'taxonomies',
+				'widgets',
+			]
+		);
+	}
+}