diff --git a/website/source/docs/other/environmental-variables.html.md b/website/source/docs/other/environmental-variables.html.md
index 35fbbb8c0..186f2160d 100644
--- a/website/source/docs/other/environmental-variables.html.md
+++ b/website/source/docs/other/environmental-variables.html.md
@@ -248,3 +248,7 @@ detection.
 
 Vagrant will not display the warning about disabling the core trigger feature if
 the community plugin is installed.
+
+## `VAGRANT_USER_AGENT_PROVISIONAL_STRING`
+
+Vagrant will append the contents of this variable to the default user agent header.