diff --git a/activerecord/lib/active_record/connection_adapters/abstract/connection_pool.rb b/activerecord/lib/active_record/connection_adapters/abstract/connection_pool.rb
index ca9314ec99..cffa2387de 100644
--- a/activerecord/lib/active_record/connection_adapters/abstract/connection_pool.rb
+++ b/activerecord/lib/active_record/connection_adapters/abstract/connection_pool.rb
@@ -1,3 +1,4 @@
+require 'thread'
 require 'monitor'
 require 'set'
 require 'active_support/core_ext/module/synchronization'
diff --git a/activerecord/test/cases/locking_test.rb b/activerecord/test/cases/locking_test.rb
index 4ddcdc010b..f9678cb0c5 100644
--- a/activerecord/test/cases/locking_test.rb
+++ b/activerecord/test/cases/locking_test.rb
@@ -1,3 +1,4 @@
+require 'thread'
 require "cases/helper"
 require 'models/person'
 require 'models/reader'
diff --git a/activesupport/lib/active_support/core_ext/module/synchronization.rb b/activesupport/lib/active_support/core_ext/module/synchronization.rb
index 38ce55f26e..ed16c2f71b 100644
--- a/activesupport/lib/active_support/core_ext/module/synchronization.rb
+++ b/activesupport/lib/active_support/core_ext/module/synchronization.rb
@@ -1,3 +1,4 @@
+require 'thread'
 require 'active_support/core_ext/module/aliasing'
 require 'active_support/core_ext/array/extract_options'
 
diff --git a/activesupport/test/core_ext/module/synchronization_test.rb b/activesupport/test/core_ext/module/synchronization_test.rb
index eb850893f0..6c407e2260 100644
--- a/activesupport/test/core_ext/module/synchronization_test.rb
+++ b/activesupport/test/core_ext/module/synchronization_test.rb
@@ -1,3 +1,4 @@
+require 'thread'
 require 'abstract_unit'
 
 require 'active_support/core_ext/class/attribute_accessors'