diff --git a/wp-includes/wp-db.php b/wp-includes/wp-db.php
index 4bba086d3f..421db3671d 100644
--- a/wp-includes/wp-db.php
+++ b/wp-includes/wp-db.php
@@ -569,12 +569,15 @@ class wpdb {
 			$this->show_errors();
 
 		/* Use ext/mysqli if it exists and:
+		 *  - USE_EXT_MYSQL is defined as false, or
 		 *  - We are a development version of WordPress, or
 		 *  - We are running PHP 5.5 or greater, or
 		 *  - ext/mysql is not loaded.
 		 */
 		if ( function_exists( 'mysqli_connect' ) ) {
-			if ( version_compare( phpversion(), '5.5', '>=' ) || ! function_exists( 'mysql_connect' ) ) {
+			if ( defined( 'USE_EXT_MYSQL' ) ) {
+				$this->use_mysqli = ! USE_EXT_MYSQL;
+			} elseif ( version_compare( phpversion(), '5.5', '>=' ) || ! function_exists( 'mysql_connect' ) ) {
 				$this->use_mysqli = true;
 			} elseif ( false !== strpos( $GLOBALS['wp_version'], '-' ) ) {
 				$this->use_mysqli = true;