diff --git a/lib/vagrant/action/builtin/box_add.rb b/lib/vagrant/action/builtin/box_add.rb
index 9003baa22..020a90eac 100644
--- a/lib/vagrant/action/builtin/box_add.rb
+++ b/lib/vagrant/action/builtin/box_add.rb
@@ -21,7 +21,9 @@ module Vagrant
           url = env[:box_url]
           if url !~ /^[a-z0-9]+:.*$/i
             @logger.info("No protocol found on box URL, assuming file:")
-            url = "file:#{File.expand_path(url)}"
+            file_path = File.expand_path(url)
+            file_path = Util::Platform.cygwin_windows_path(file_path)
+            url = "file:#{file_path}"
           end
 
           downloader_options = {}