diff --git a/guides/source/testing.md b/guides/source/testing.md
index 30d5bc9cf3..1187b6c183 100644
--- a/guides/source/testing.md
+++ b/guides/source/testing.md
@@ -535,7 +535,7 @@ class ActiveSupport::TestCase
 end
 ```
 
-Rails applications generated from JRuby will automatically include the `with: :threads` option.
+Rails applications generated from JRuby or TruffleRuby will automatically include the `with: :threads` option.
 
 The number of workers passed to `parallelize` determines the number of threads the tests will use. You may
 want to parallelize your local test suite differently from your CI, so an environment variable is provided
diff --git a/railties/lib/rails/generators/rails/app/templates/test/test_helper.rb.tt b/railties/lib/rails/generators/rails/app/templates/test/test_helper.rb.tt
index cc677ab411..472c9a7739 100644
--- a/railties/lib/rails/generators/rails/app/templates/test/test_helper.rb.tt
+++ b/railties/lib/rails/generators/rails/app/templates/test/test_helper.rb.tt
@@ -4,10 +4,10 @@ require "rails/test_help"
 
 class ActiveSupport::TestCase
   # Run tests in parallel with specified workers
-<% if defined?(JRUBY_VERSION) || Gem.win_platform? -%>
-  parallelize(workers: :number_of_processors, with: :threads)
-<%- else -%>
+<% if Process.respond_to?(:fork) && !Gem.win_platform? -%>
   parallelize(workers: :number_of_processors)
+<%- else -%>
+  parallelize(workers: :number_of_processors, with: :threads)
 <% end -%>
 
 <% unless options[:skip_active_record] -%>
diff --git a/railties/test/generators/app_generator_test.rb b/railties/test/generators/app_generator_test.rb
index 3277b70406..94baa2d7ab 100644
--- a/railties/test/generators/app_generator_test.rb
+++ b/railties/test/generators/app_generator_test.rb
@@ -862,7 +862,7 @@ def test_spring_binstubs
 
   def test_spring_no_fork
     jruby_skip "spring doesn't run on JRuby"
-    assert_called_with(Process, :respond_to?, [[:fork], [:fork], [:fork]], returns: false) do
+    assert_called_with(Process, :respond_to?, [[:fork], [:fork], [:fork], [:fork]], returns: false) do
       run_generator
 
       assert_no_gem "spring"