diff --git a/sphinx/builders/linkcheck.py b/sphinx/builders/linkcheck.py
index ef8f9d90222..9b54afc7ca7 100644
--- a/sphinx/builders/linkcheck.py
+++ b/sphinx/builders/linkcheck.py
@@ -35,7 +35,7 @@
 
 logger = logging.getLogger(__name__)
 
-uri_re = re.compile('[a-z]+://')
+uri_re = re.compile('([a-z]+:)?//')  # matches to foo:// and // (a protocol relative URL)
 
 
 DEFAULT_REQUEST_HEADERS = {