diff --git a/backport-changelog/6.6/6756.md b/backport-changelog/6.6/6756.md
index e60e128bf45cb..688b258c095fa 100644
--- a/backport-changelog/6.6/6756.md
+++ b/backport-changelog/6.6/6756.md
@@ -1,3 +1,4 @@
 https://github.com/WordPress/wordpress-develop/pull/6756
 
 * https://github.com/WordPress/gutenberg/pull/62461
+* https://github.com/WordPress/gutenberg/pull/62495
diff --git a/lib/class-wp-rest-global-styles-controller-gutenberg.php b/lib/class-wp-rest-global-styles-controller-gutenberg.php
index 9bbca1ffcab40..aadcb2fd7de1e 100644
--- a/lib/class-wp-rest-global-styles-controller-gutenberg.php
+++ b/lib/class-wp-rest-global-styles-controller-gutenberg.php
@@ -330,6 +330,25 @@ protected function prepare_item_for_database( $request ) {
 			} elseif ( isset( $existing_config['styles'] ) ) {
 				$config['styles'] = $existing_config['styles'];
 			}
+
+			/*
+			 * If the incoming request is going to create a new variation
+			 * that is not yet registered, we register it here.
+			 * This is because the variations are registered on init,
+			 * but we want this endpoint to return the new variation immediately:
+			 * if we don't register it, it'll be stripped out of the response
+			 * just in this request (subsequent ones will be ok).
+			 * Take the variations defined in styles.blocks.variations from the incoming request
+			 * that are not part of the $existing_config.
+			 */
+			if ( isset( $request['styles']['blocks']['variations'] ) ) {
+				$existing_variations = isset( $existing_config['styles']['blocks']['variations'] ) ? $existing_config['styles']['blocks']['variations'] : array();
+				$new_variations      = array_diff_key( $request['styles']['blocks']['variations'], $existing_variations );
+				if ( ! empty( $new_variations ) ) {
+					gutenberg_register_block_style_variations_from_theme_json_data( $new_variations );
+				}
+			}
+
 			if ( isset( $request['settings'] ) ) {
 				$config['settings'] = $request['settings'];
 			} elseif ( isset( $existing_config['settings'] ) ) {
diff --git a/phpunit/class-wp-rest-global-styles-controller-gutenberg-test.php b/phpunit/class-wp-rest-global-styles-controller-gutenberg-test.php
index f5b216a084a4c..66a236c1c40a9 100644
--- a/phpunit/class-wp-rest-global-styles-controller-gutenberg-test.php
+++ b/phpunit/class-wp-rest-global-styles-controller-gutenberg-test.php
@@ -513,6 +513,56 @@ public function test_update_item_invalid_styles_css() {
 		$this->assertErrorResponse( 'rest_custom_css_illegal_markup', $response, 400 );
 	}
 
+	/**
+	 * Tests the submission of a custom block style variation that was defined
+	 * within a theme style variation and wouldn't be registered at the time
+	 * of saving via the API.
+	 *
+	 * @since 6.6.0
+	 *
+	 * @covers WP_REST_Global_Styles_Controller_Gutenberg::update_item
+	 */
+	public function test_update_item_with_custom_block_style_variations() {
+		wp_set_current_user( self::$admin_id );
+		if ( is_multisite() ) {
+			grant_super_admin( self::$admin_id );
+		}
+
+		$group_variations = array(
+			'fromThemeStyleVariation' => array(
+				'color' => array(
+					'background' => '#ffffff',
+					'text'       => '#000000',
+				),
+			),
+		);
+
+		$request = new WP_REST_Request( 'PUT', '/wp/v2/global-styles/' . self::$global_styles_id );
+		$request->set_body_params(
+			array(
+				'styles' => array(
+					'blocks' => array(
+						'variations' => array(
+							'fromThemeStyleVariation' => array(
+								'blockTypes' => array( 'core/group', 'core/columns' ),
+								'color'      => array(
+									'background' => '#000000',
+									'text'       => '#ffffff',
+								),
+							),
+						),
+						'core/group' => array(
+							'variations' => $group_variations,
+						),
+					),
+				),
+			)
+		);
+		$response = rest_get_server()->dispatch( $request );
+		$data     = $response->get_data();
+		$this->assertSame( $group_variations, $data['styles']['blocks']['core/group']['variations'] );
+	}
+
 	/**
 	 * @doesNotPerformAssertions
 	 */