diff --git a/lib/compat/wordpress-5.9/class-gutenberg-rest-templates-controller.php b/lib/compat/wordpress-5.9/class-gutenberg-rest-templates-controller.php
index eb7f047a8e98df..12aa2124da3731 100644
--- a/lib/compat/wordpress-5.9/class-gutenberg-rest-templates-controller.php
+++ b/lib/compat/wordpress-5.9/class-gutenberg-rest-templates-controller.php
@@ -414,6 +414,10 @@ public function prepare_item_for_response( $template, $request ) { // phpcs:igno
 			'has_theme_file' => $template->has_theme_file,
 		);
 
+		if ( 'wp_template' === $template->type ) {
+			$result['is_custom'] = $template->is_custom;
+		}
+
 		if ( 'wp_template_part' === $template->type ) {
 			$result['area'] = $template->area;
 		}
@@ -582,6 +586,15 @@ public function get_item_schema() {
 			),
 		);
 
+		if ( 'wp_template' === $this->post_type ) {
+			$schema['properties']['is_custom'] = array(
+				'description' => __( 'Whether a template is a custom template.', 'gutenberg' ),
+				'type'        => 'bool',
+				'context'     => array( 'embed', 'view', 'edit' ),
+				'readonly'    => true,
+			);
+		}
+
 		if ( 'wp_template_part' === $this->post_type ) {
 			$schema['properties']['area'] = array(
 				'description' => __( 'Where the template part is intended for use (header, footer, etc.)', 'gutenberg' ),
diff --git a/phpunit/class-gutenberg-rest-template-controller-test.php b/phpunit/class-gutenberg-rest-template-controller-test.php
index 4fc3b5407e06e9..6c6d62be1b417d 100644
--- a/phpunit/class-gutenberg-rest-template-controller-test.php
+++ b/phpunit/class-gutenberg-rest-template-controller-test.php
@@ -76,6 +76,7 @@ function find_and_normalize_template_by_id( $templates, $id ) {
 				'type'           => 'wp_template',
 				'wp_id'          => null,
 				'has_theme_file' => true,
+				'is_custom'      => false,
 			),
 			find_and_normalize_template_by_id( $data, 'tt1-blocks//index' )
 		);
@@ -129,6 +130,7 @@ public function test_get_item() {
 				'type'           => 'wp_template',
 				'wp_id'          => null,
 				'has_theme_file' => true,
+				'is_custom'      => false,
 			),
 			$data
 		);
@@ -193,6 +195,7 @@ public function test_create_item() {
 					'raw' => 'Content',
 				),
 				'has_theme_file' => false,
+				'is_custom'      => true,
 			),
 			$data
 		);