diff --git a/Library/Homebrew/dev-cmd/tests.rb b/Library/Homebrew/dev-cmd/tests.rb
index a6f54d9d60..1da41139bc 100644
--- a/Library/Homebrew/dev-cmd/tests.rb
+++ b/Library/Homebrew/dev-cmd/tests.rb
@@ -42,7 +42,6 @@ module Homebrew
     args = tests_args.parse
 
     Homebrew.install_bundler_gems!
-    gem_user_dir = Gem.user_dir
 
     require "byebug" if args.byebug?
 
@@ -138,6 +137,9 @@ module Homebrew
 
       puts "Randomized with seed #{seed}"
 
+      # Let tests find `bundle` in the actual location.
+      ENV["HOMEBREW_TESTS_GEM_USER_DIR"] = gem_user_dir
+
       # Let `bundle` in PATH find its gem.
       ENV["GEM_PATH"] = "#{ENV["GEM_PATH"]}:#{gem_user_dir}"
 
diff --git a/Library/Homebrew/utils/gems.rb b/Library/Homebrew/utils/gems.rb
index eb3a5f073a..a75f0b8561 100644
--- a/Library/Homebrew/utils/gems.rb
+++ b/Library/Homebrew/utils/gems.rb
@@ -17,9 +17,13 @@ module Homebrew
     "#{RbConfig::CONFIG["prefix"]}/bin"
   end
 
+  def gem_user_dir
+    ENV["HOMEBREW_TESTS_GEM_USER_DIR"] || Gem.user_dir
+  end
+
   def gem_user_bindir
     require "rubygems"
-    "#{Gem.user_dir}/bin"
+    "#{gem_user_dir}/bin"
   end
 
   def ohai_if_defined(message)
@@ -55,7 +59,7 @@ module Homebrew
     Gem.clear_paths
     Gem::Specification.reset
 
-    # Add necessary Ruby and Gem binary directories to PATH.
+    # Add necessary Ruby and Gem binary directories to `PATH`.
     gem_bindir ||= Gem.bindir
     paths = ENV.fetch("PATH").split(":")
     paths.unshift(gem_bindir) unless paths.include?(gem_bindir)
@@ -92,7 +96,7 @@ module Homebrew
 
   def install_bundler!
     require "rubygems"
-    setup_gem_environment!(gem_home: Gem.user_dir, gem_bindir: gem_user_bindir)
+    setup_gem_environment!(gem_home: gem_user_dir, gem_bindir: gem_user_bindir)
     install_gem_setup_path!(
       "bundler",
       version:               HOMEBREW_BUNDLER_VERSION,