diff --git src/wp-includes/rest-api/rest-functions.php src/wp-includes/rest-api/rest-functions.php
index 7964bae..280967f 100644
--- src/wp-includes/rest-api/rest-functions.php
+++ src/wp-includes/rest-api/rest-functions.php
@@ -20,11 +20,25 @@
  *                          multiple methods. Default empty array.
  * @param bool   $override  Optional. If the route already exists, should we override it? True overrides,
  *                          false merges (with newer overriding if duplicate keys exist). Default false.
+ * @return bool True on success, false on error.
  */
 function register_rest_route( $namespace, $route, $args = array(), $override = false ) {
 	/** @var WP_REST_Server $wp_rest_server */
 	global $wp_rest_server;
 
+	if ( empty( $namespace ) ) {
+		/*
+		 * Non-namespaced routes are not allowed, with the exception of the main
+		 * and namespace indexes. If you really need to register a
+		 * non-namespaced route, call `WP_REST_Server::register_route` directly.
+		 */
+		_doing_it_wrong( 'register_rest_route', 'Routes must be namespaced with plugin or theme name and version', '4.4.0' );
+		return false;
+	} else if ( empty( $route ) ) {
+		_doing_it_wrong( 'register_rest_route', 'Route must be specified', '4.4.0' );
+		return false;
+	}
+
 	if ( isset( $args['callback'] ) ) {
 		// Upgrade a single set to multiple.
 		$args = array( $args );
@@ -44,20 +58,9 @@ function register_rest_route( $namespace, $route, $args = array(), $override = f
 		$arg_group = array_merge( $defaults, $arg_group );
 	}
 
-	if ( $namespace ) {
-		$full_route = '/' . trim( $namespace, '/' ) . '/' . trim( $route, '/' );
-	} else {
-		/*
-		 * Non-namespaced routes are not allowed, with the exception of the main
-		 * and namespace indexes. If you really need to register a
-		 * non-namespaced route, call `WP_REST_Server::register_route` directly.
-		 */
-		_doing_it_wrong( 'register_rest_route', 'Routes must be namespaced with plugin name and version', '4.4.0' );
-
-		$full_route = '/' . trim( $route, '/' );
-	}
-
+	$full_route = '/' . trim( $namespace, '/' ) . '/' . trim( $route, '/' );
 	$wp_rest_server->register_route( $namespace, $full_route, $args, $override );
+	return true;
 }
 
 /**
diff --git tests/phpunit/tests/rest-api.php tests/phpunit/tests/rest-api.php
index ff7e5cd..f417a93 100644
--- tests/phpunit/tests/rest-api.php
+++ tests/phpunit/tests/rest-api.php
@@ -149,6 +149,34 @@ class Tests_REST_API extends WP_UnitTestCase {
 	}
 
 	/**
+	 * Test that we reject routes without namespaces
+	 *
+	 * @expectedIncorrectUsage register_rest_route
+	 */
+	public function test_route_reject_empty_namespace() {
+		register_rest_route( '', '/test-empty-namespace', array(
+			'methods'      => array( 'POST' ),
+			'callback'     => '__return_null',
+		), true );
+		$endpoints = $GLOBALS['wp_rest_server']->get_routes();
+		$this->assertFalse( isset( $endpoints['/test-empty-namespace'] ) );
+	}
+
+	/**
+	 * Test that we reject empty routes
+	 *
+	 * @expectedIncorrectUsage register_rest_route
+	 */
+	public function test_route_reject_empty_route() {
+		register_rest_route( '/test-empty-route', '', array(
+			'methods'      => array( 'POST' ),
+			'callback'     => '__return_null',
+		), true );
+		$endpoints = $GLOBALS['wp_rest_server']->get_routes();
+		$this->assertFalse( isset( $endpoints['/test-empty-route'] ) );
+	}
+
+	/**
 	 * The rest_route query variable should be registered.
 	 */
 	function test_rest_route_query_var() {