diff --git a/src/wp-includes/rest-api/endpoints/class-wp-rest-themes-controller.php b/src/wp-includes/rest-api/endpoints/class-wp-rest-themes-controller.php
index 71dbcb6193..075be9efe4 100644
--- a/src/wp-includes/rest-api/endpoints/class-wp-rest-themes-controller.php
+++ b/src/wp-includes/rest-api/endpoints/class-wp-rest-themes-controller.php
@@ -326,6 +326,10 @@ class WP_REST_Themes_Controller extends WP_REST_Controller {
 			}
 		}
 
+		if ( rest_is_field_included( 'is_block_theme', $fields ) ) {
+			$data['is_block_theme'] = $theme->is_block_theme();
+		}
+
 		$data = $this->add_additional_fields_to_object( $data, $request );
 
 		// Wrap the data in a response object.
@@ -494,6 +498,11 @@ class WP_REST_Themes_Controller extends WP_REST_Controller {
 						),
 					),
 				),
+				'is_block_theme' => array(
+					'description' => __( 'Whether the theme is a block-based theme.' ),
+					'type'        => 'boolean',
+					'readonly'    => true,
+				),
 				'name'           => array(
 					'description' => __( 'The name of the theme.' ),
 					'type'        => 'object',
diff --git a/tests/phpunit/tests/rest-api/rest-themes-controller.php b/tests/phpunit/tests/rest-api/rest-themes-controller.php
index 01e1a2b6ad..24b564debb 100644
--- a/tests/phpunit/tests/rest-api/rest-themes-controller.php
+++ b/tests/phpunit/tests/rest-api/rest-themes-controller.php
@@ -172,6 +172,7 @@ class WP_Test_REST_Themes_Controller extends WP_Test_REST_Controller_Testcase {
 			'author',
 			'author_uri',
 			'description',
+			'is_block_theme',
 			'name',
 			'requires_php',
 			'requires_wp',
@@ -185,6 +186,8 @@ class WP_Test_REST_Themes_Controller extends WP_Test_REST_Controller_Testcase {
 			'theme_uri',
 			'version',
 		);
+		$this->assertIsArray( $data );
+		$this->assertNotEmpty( $data );
 		$this->assertSameSets( $fields, array_keys( $data[0] ) );
 	}
 
@@ -208,6 +211,7 @@ class WP_Test_REST_Themes_Controller extends WP_Test_REST_Controller_Testcase {
 			'author',
 			'author_uri',
 			'description',
+			'is_block_theme',
 			'name',
 			'requires_php',
 			'requires_wp',
@@ -220,6 +224,8 @@ class WP_Test_REST_Themes_Controller extends WP_Test_REST_Controller_Testcase {
 			'theme_uri',
 			'version',
 		);
+		$this->assertIsArray( $data );
+		$this->assertNotEmpty( $data );
 		$this->assertSameSets( $fields, array_keys( $data[0] ) );
 
 		$this->assertContains( 'twentytwenty', wp_list_pluck( $data, 'stylesheet' ) );
@@ -343,7 +349,7 @@ class WP_Test_REST_Themes_Controller extends WP_Test_REST_Controller_Testcase {
 		$response   = self::perform_active_theme_request( 'OPTIONS' );
 		$data       = $response->get_data();
 		$properties = $data['schema']['properties'];
-		$this->assertCount( 15, $properties );
+		$this->assertCount( 16, $properties );
 
 		$this->assertArrayHasKey( 'author', $properties );
 		$this->assertArrayHasKey( 'raw', $properties['author']['properties'] );
@@ -357,6 +363,8 @@ class WP_Test_REST_Themes_Controller extends WP_Test_REST_Controller_Testcase {
 		$this->assertArrayHasKey( 'raw', $properties['description']['properties'] );
 		$this->assertArrayHasKey( 'rendered', $properties['description']['properties'] );
 
+		$this->assertArrayHasKey( 'is_block_theme', $properties );
+
 		$this->assertArrayHasKey( 'name', $properties );
 		$this->assertArrayHasKey( 'raw', $properties['name']['properties'] );
 		$this->assertArrayHasKey( 'rendered', $properties['name']['properties'] );
@@ -471,6 +479,27 @@ class WP_Test_REST_Themes_Controller extends WP_Test_REST_Controller_Testcase {
 		$this->assertSame( '5.3', $result[0]['requires_wp'] );
 	}
 
+	/**
+	 * @ticket 58123
+	 * @covers WP_REST_Themes_Controller::prepare_item_for_response
+	 */
+	public function test_theme_is_block_theme() {
+		// Test classic theme, activated in test setup.
+		$response = self::perform_active_theme_request();
+		$result   = $response->get_data();
+
+		$this->assertArrayHasKey( 'is_block_theme', $result[0] );
+		$this->assertFalse( $result[0]['is_block_theme'] );
+
+		// Test block theme.
+		switch_theme( 'block-theme' );
+		$response = self::perform_active_theme_request();
+		$result   = $response->get_data();
+
+		$this->assertArrayHasKey( 'is_block_theme', $result[0] );
+		$this->assertTrue( $result[0]['is_block_theme'] );
+	}
+
 	/**
 	 * @ticket 49906
 	 */
@@ -1234,6 +1263,7 @@ class WP_Test_REST_Themes_Controller extends WP_Test_REST_Controller_Testcase {
 			'author',
 			'author_uri',
 			'description',
+			'is_block_theme',
 			'name',
 			'requires_php',
 			'requires_wp',