diff --git a/src/wp-includes/js/api-request.js b/src/wp-includes/js/api-request.js
index f0a84ef48f..4b299beb69 100644
--- a/src/wp-includes/js/api-request.js
+++ b/src/wp-includes/js/api-request.js
@@ -22,7 +22,7 @@
 	apiRequest.buildAjaxOptions = function( options ) {
 		var url = options.url;
 		var path = options.path;
-		var namespaceTrimmed, endpointTrimmed;
+		var namespaceTrimmed, endpointTrimmed, apiRoot;
 		var headers, addNonceHeader, headerName;
 
 		if (
@@ -38,7 +38,16 @@
 			}
 		}
 		if ( typeof path === 'string' ) {
-			url = wpApiSettings.root + path.replace( /^\//, '' );
+			apiRoot = wpApiSettings.root;
+			path = path.replace( /^\//, '' );
+
+			// API root may already include query parameter prefix if site is
+			// configured to use plain permalinks.
+			if ( 'string' === typeof apiRoot && -1 !== apiRoot.indexOf( '?' ) ) {
+				path = path.replace( '?', '&' );
+			}
+
+			url = apiRoot + path;
 		}
 
 		// If ?_wpnonce=... is present, no need to add a nonce header.
diff --git a/tests/qunit/wp-includes/js/api-request.js b/tests/qunit/wp-includes/js/api-request.js
index 99a1213ea3..86d0d72cda 100644
--- a/tests/qunit/wp-includes/js/api-request.js
+++ b/tests/qunit/wp-includes/js/api-request.js
@@ -140,9 +140,9 @@
 			window.wpApiSettings.root = 'http://localhost/index.php?rest_route=/';
 			assert.deepEqual( wp.apiRequest.buildAjaxOptions( {
 				namespace: '/wp/v2/',
-				endpoint: '/posts'
+				endpoint: '/posts?orderby=title'
 			} ), {
-				url: 'http://localhost/index.php?rest_route=/wp/v2/posts',
+				url: 'http://localhost/index.php?rest_route=/wp/v2/posts&orderby=title',
 				headers: nonceHeader
 			} );
 		}