Index: src/wp-admin/includes/class-wp-press-this.php
===================================================================
--- src/wp-admin/includes/class-wp-press-this.php	(revision 31793)
+++ src/wp-admin/includes/class-wp-press-this.php	(working copy)
@@ -928,6 +928,19 @@
 			$link = $data['_links']['shortlink'];
 		}
 
+		if ( ! preg_match( '/^https?:/', $link ) ) {
+			if ( preg_match( '/^[\/]{1}[^\/]+/', $link ) ) {
+				// From root: /blah
+				$link = preg_replace( '/^(https?:\/\/[^\/]+)(\/.*)?$/', '\1', $data['u'] ) . $link;
+			} else if ( preg_match( '/^[\/]{2}[^\/]+/', $link ) ) {
+				// Protocol relative: //blah.com/blah
+				$link = preg_replace( '/^(https?:)\/\/.+$/', '\1', $data['u'] ) . $link;
+			} else {
+				// Relative, or diff protocol
+				return '';
+			}
+		}
+
 		return $link;
 	}