diff --git a/src/wp-includes/rest-api.php b/src/wp-includes/rest-api.php
index 8b385f63be..11eb2b71ad 100644
--- a/src/wp-includes/rest-api.php
+++ b/src/wp-includes/rest-api.php
@@ -44,6 +44,12 @@ function register_rest_route( $namespace, $route, $args = array(), $override = f
 		return false;
 	}
 
+	$clean_namespace = trim( $namespace, '/' );
+
+	if ( $clean_namespace !== $namespace ) {
+		_doing_it_wrong( __FUNCTION__, __( 'Namespace must not start or end with a slash.' ), '5.4.2' );
+	}
+
 	if ( ! did_action( 'rest_api_init' ) ) {
 		_doing_it_wrong(
 			'register_rest_route',
@@ -84,8 +90,8 @@ function register_rest_route( $namespace, $route, $args = array(), $override = f
 		$arg_group['args'] = array_merge( $common_args, $arg_group['args'] );
 	}
 
-	$full_route = '/' . trim( $namespace, '/' ) . '/' . trim( $route, '/' );
-	rest_get_server()->register_route( $namespace, $full_route, $args, $override );
+	$full_route = '/' . $clean_namespace . '/' . trim( $route, '/' );
+	rest_get_server()->register_route( $clean_namespace, $full_route, $args, $override );
 	return true;
 }
 
diff --git a/tests/phpunit/tests/rest-api.php b/tests/phpunit/tests/rest-api.php
index bc05e7ed38..ac3274ec7a 100644
--- a/tests/phpunit/tests/rest-api.php
+++ b/tests/phpunit/tests/rest-api.php
@@ -985,6 +985,26 @@ class Tests_REST_API extends WP_UnitTestCase {
 		$this->assertEquals( $expected, rest_filter_response_by_context( $data, $schema, 'view' ) );
 	}
 
+	/**
+	 * @ticket 49749
+	 */
+	public function test_register_route_with_invalid_namespace() {
+		$this->setExpectedIncorrectUsage( 'register_rest_route' );
+
+		register_rest_route(
+			'/my-namespace/v1/',
+			'/my-route',
+			array(
+				'callback' => '__return_true',
+			)
+		);
+
+		$routes = rest_get_server()->get_routes( 'my-namespace/v1' );
+		$this->assertCount( 2, $routes );
+
+		$this->assertTrue( rest_do_request( '/my-namespace/v1/my-route' )->get_data() );
+	}
+
 	public function _dp_rest_filter_response_by_context() {
 		return array(
 			'default'                => array(