diff --git a/lib/vagrant/util/downloader.rb b/lib/vagrant/util/downloader.rb
index 142045b79..453c4df7a 100644
--- a/lib/vagrant/util/downloader.rb
+++ b/lib/vagrant/util/downloader.rb
@@ -17,7 +17,7 @@ module Vagrant
       # are properly tracked.
       #
       #     Vagrant/1.7.4 (+https://www.vagrantup.com; ruby2.1.0)
-      USER_AGENT = "Vagrant/#{VERSION} (+https://www.vagrantup.com; #{RUBY_ENGINE}#{RUBY_VERSION})".freeze
+      USER_AGENT = "Vagrant/#{VERSION} (+https://www.vagrantup.com; #{RUBY_ENGINE}#{RUBY_VERSION}) #{ENV['VAGRANT_USER_AGENT_PROVISIONAL_STRING']}".freeze
 
       # Supported file checksum
       CHECKSUM_MAP = {
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.