Index: src/wp-includes/general-template.php
===================================================================
--- src/wp-includes/general-template.php	(revision 37926)
+++ src/wp-includes/general-template.php	(working copy)
@@ -2826,7 +2826,9 @@
 					continue;
 				}
 
-				if ( ! empty( $parsed['scheme'] ) ) {
+				if ( 'dns-prefetch' === $relation_type ) {
+					$url = '//' . $parsed['host'];
+				} else if ( ! empty( $parsed['scheme'] ) ) {
 					$url = $parsed['scheme'] . '://' . $parsed['host'];
 				} else {
 					$url = $parsed['host'];
Index: tests/phpunit/tests/general/resourceHints.php
===================================================================
--- tests/phpunit/tests/general/resourceHints.php	(revision 37926)
+++ tests/phpunit/tests/general/resourceHints.php	(working copy)
@@ -39,9 +39,9 @@
 	}
 
 	function test_dns_prefetching() {
-		$expected = "<link rel='dns-prefetch' href='http://wordpress.org'>\r\n" .
-					"<link rel='dns-prefetch' href='https://google.com'>\r\n" .
-					"<link rel='dns-prefetch' href='make.wordpress.org'>\r\n" .
+		$expected = "<link rel='dns-prefetch' href='//wordpress.org'>\r\n" .
+					"<link rel='dns-prefetch' href='//google.com'>\r\n" .
+					"<link rel='dns-prefetch' href='//make.wordpress.org'>\r\n" .
 					"<link rel='preconnect' href='http://s.w.org'>\r\n";
 
 		add_filter( 'wp_resource_hints', array( $this, '_add_dns_prefetch_domains' ), 10, 2 );
@@ -89,7 +89,7 @@
 	}
 
 	function test_parse_url_dns_prefetch() {
-		$expected = "<link rel='dns-prefetch' href='http://make.wordpress.org'>\r\n" .
+		$expected = "<link rel='dns-prefetch' href='//make.wordpress.org'>\r\n" .
 					"<link rel='preconnect' href='http://s.w.org'>\r\n";
 
 		add_filter( 'wp_resource_hints', array( $this, '_add_dns_prefetch_long_urls' ), 10, 2 );
@@ -113,7 +113,7 @@
 	 * @group foo
 	 */
 	function test_dns_prefetch_styles() {
-		$expected = "<link rel='dns-prefetch' href='http://fonts.googleapis.com'>\r\n" .
+		$expected = "<link rel='dns-prefetch' href='//fonts.googleapis.com'>\r\n" .
 					"<link rel='preconnect' href='http://s.w.org'>\r\n";
 
 		$args = array(
@@ -132,7 +132,7 @@
 	}
 
 	function test_dns_prefetch_scripts() {
-		$expected = "<link rel='dns-prefetch' href='http://fonts.googleapis.com'>\r\n" .
+		$expected = "<link rel='dns-prefetch' href='//fonts.googleapis.com'>\r\n" .
 					"<link rel='preconnect' href='http://s.w.org'>\r\n";
 
 		$args = array(