diff --git a/wp-includes/load.php b/wp-includes/load.php
index e589f5fa5a..48cd487873 100644
--- a/wp-includes/load.php
+++ b/wp-includes/load.php
@@ -107,7 +107,7 @@ function wp_check_php_mysql_versions() {
 		die( sprintf( __( 'Your server is running PHP version %1$s but WordPress %2$s requires at least %3$s.' ), $php_version, $wp_version, $required_php_version ) );
 	}
 
-	if ( ! extension_loaded( 'mysql' ) && ! file_exists( WP_CONTENT_DIR . '/db.php' ) ) {
+	if ( ! extension_loaded( 'mysql' ) && ! extension_loaded( 'mysqli' ) && ! file_exists( WP_CONTENT_DIR . '/db.php' ) ) {
 		wp_load_translations_early();
 		die( __( 'Your PHP installation appears to be missing the MySQL extension which is required by WordPress.' ) );
 	}
diff --git a/wp-includes/wp-db.php b/wp-includes/wp-db.php
index dabce8cdfe..667ee3b25a 100644
--- a/wp-includes/wp-db.php
+++ b/wp-includes/wp-db.php
@@ -568,7 +568,18 @@ class wpdb {
 		if ( WP_DEBUG && WP_DEBUG_DISPLAY )
 			$this->show_errors();
 
-		$this->use_mysqli = ( version_compare( phpversion(), '5.5', '>=' ) && function_exists( 'mysqli_connect' ) );
+		/* Use ext/mysqli if it exists and:
+		 *  - 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' ) ) {
+				$this->use_mysqli = true;
+			} elseif ( false !== strpos( $GLOBALS['wp_version'], '-' ) ) {
+				$this->use_mysqli = true;
+			}
+		}
 
 		$this->init_charset();