diff --git a/bundler/spec/support/path.rb b/bundler/spec/support/path.rb
index cc750f55d81f..9f634a5e86e4 100644
--- a/bundler/spec/support/path.rb
+++ b/bundler/spec/support/path.rb
@@ -114,7 +114,7 @@ def tmp(*path)
     end
 
     def tmp_root
-      source_root.join("tmp")
+      ruby_core? && (tmpdir = ENV["TMPDIR"]) ? Pathname(tmpdir) : source_root.join("tmp")
     end
 
     # Bump this version whenever you make a breaking change to the spec setup
diff --git a/bundler/spec/support/rubygems_ext.rb b/bundler/spec/support/rubygems_ext.rb
index 2d681529aac2..997a70747d20 100644
--- a/bundler/spec/support/rubygems_ext.rb
+++ b/bundler/spec/support/rubygems_ext.rb
@@ -43,7 +43,7 @@ def test_setup
       # sign extension bundles on macOS, to avoid trying to find the specified key
       # from the fake $HOME/Library/Keychains directory.
       ENV.delete "RUBY_CODESIGN"
-      ENV["TMPDIR"] = Path.tmpdir.to_s
+      ENV["TMPDIR"] = Path.tmpdir.to_s unless ruby_core?
 
       require "rubygems/user_interaction"
       Gem::DefaultUserInteraction.ui = Gem::SilentUI.new