diff --git src/wp-admin/js/customize-controls.js src/wp-admin/js/customize-controls.js
index 4bfc252..831a9e6 100644
--- src/wp-admin/js/customize-controls.js
+++ src/wp-admin/js/customize-controls.js
@@ -1128,6 +1128,13 @@
 			});
 		});
 
+		// Prompt user with AYS dialog if leaving the Customizer with unsaved changes
+		$( window ).on( 'beforeunload', function () {
+			if ( ! api.state( 'saved' )() ) {
+				return api.l10n.saveAlert;
+			}
+		} );
+
 		// Pass events through to the parent.
 		api.bind( 'saved', function() {
 			parent.send( 'saved' );
diff --git src/wp-includes/script-loader.php src/wp-includes/script-loader.php
index df42718..c0d42b6 100644
--- src/wp-includes/script-loader.php
+++ src/wp-includes/script-loader.php
@@ -381,6 +381,7 @@ function wp_default_scripts( &$scripts ) {
 	did_action( 'init' ) && $scripts->localize( 'customize-controls', '_wpCustomizeControlsL10n', array(
 		'activate'  => __( 'Save & Activate' ),
 		'save'      => __( 'Save & Publish' ),
+		'saveAlert' => __( 'The changes you made will be lost if you navigate away from this page.' ),
 		'saved'     => __( 'Saved' ),
 		'cancel'    => __( 'Cancel' ),
 		'close'     => __( 'Close' ),