diff --git a/doc/RUNNING_TESTS b/doc/RUNNING_TESTS
index 35e271a34..fa4737e11 100644
--- a/doc/RUNNING_TESTS
+++ b/doc/RUNNING_TESTS
@@ -14,6 +14,10 @@ Apache perl module Redmine.pm and Capybara tests, see below).
 You can run `ruby test/unit/issue_test.rb` for running a single test case and
 `ruby test/unit/issue_test.rb -n test_create` for running a single test.
 
+You can run tests in parallel by setting the PARALLEL_WORKERS environment
+variable:
+`PARALLEL_WORKERS=8 rake test`
+
 Before running tests, you need to configure both development
 and test databases.
 
diff --git a/test/test_helper.rb b/test/test_helper.rb
index 10f4f6e35..95b877d44 100644
--- a/test/test_helper.rb
+++ b/test/test_helper.rb
@@ -53,6 +53,8 @@ class ActionView::TestCase
 end
 
 class ActiveSupport::TestCase
+  parallelize(workers: 1)
+
   include ActionDispatch::TestProcess
 
   self.use_transactional_tests = true