diff --git a/src/wp-includes/formatting.php b/src/wp-includes/formatting.php
index 690b95a84d..240a21a928 100644
--- a/src/wp-includes/formatting.php
+++ b/src/wp-includes/formatting.php
@@ -2259,8 +2259,23 @@ function wp_rel_nofollow( $text ) {
  */
 function wp_rel_nofollow_callback( $matches ) {
 	$text = $matches[1];
-	$text = str_replace(array(' rel="nofollow"', " rel='nofollow'"), '', $text);
-	return "<a $text rel=\"nofollow\">";
+	$atts = shortcode_parse_atts( $matches[1] );
+	$rel = 'nofollow';
+	if ( ! empty( $atts['rel'] ) ) {
+		$parts = array_map( 'trim', explode( ' ', $atts['rel'] ) );
+		if ( false === array_search( 'nofollow', $parts ) ) {
+			$parts[] = 'nofollow';
+		}
+		$rel = implode( ' ', $parts );
+		unset( $atts['rel'] );
+
+		$html = '';
+		foreach ( $atts as $name => $value ) {
+			$html .= "{$name}=\"$value\" ";
+		}
+		$text = trim( $html );
+	}
+	return "<a $text rel=\"$rel\">";
 }
 
 /**
diff --git a/tests/phpunit/tests/formatting/WPRelNoFollow.php b/tests/phpunit/tests/formatting/WPRelNoFollow.php
new file mode 100644
index 0000000000..8d6a514e8e
--- /dev/null
+++ b/tests/phpunit/tests/formatting/WPRelNoFollow.php
@@ -0,0 +1,18 @@
+<?php
+/**
+ * @ticket 9959
+ */
+class Tests_Rel_No_Follow extends WP_UnitTestCase {
+
+	public function test_add_no_follow() {
+		$content = '<p>This is some cool <a href="/">Code</a></p>';
+		$expected = '<p>This is some cool <a href=\"/\" rel=\"nofollow\">Code</a></p>';
+		$this->assertEquals( $expected, wp_rel_nofollow( $content ) );
+	}
+
+	public function test_convert_no_follow() {
+		$content = '<p>This is some cool <a href="/" rel="weird">Code</a></p>';
+		$expected = '<p>This is some cool <a href=\"/\" rel=\"weird nofollow\">Code</a></p>';
+		$this->assertEquals( $expected, wp_rel_nofollow( $content ) );
+	}
+}
\ No newline at end of file