diff --git a/lib/thwait.gemspec b/lib/thwait.gemspec
new file mode 100644
index 0000000000..9a0355f0b5
--- /dev/null
+++ b/lib/thwait.gemspec
@@ -0,0 +1,21 @@
+require_relative "thwait/version"
+
+Gem::Specification.new do |spec|
+  spec.name          = "thwait"
+  spec.version       = ThreadsWait::VERSION
+  spec.authors       = ["Keiju ISHITSUKA"]
+  spec.email         = ["keiju@ruby-lang.org"]
+
+  spec.summary       = %q{Watches for termination of multiple threads.}
+  spec.description   = %q{Watches for termination of multiple threads.}
+  spec.homepage      = "https://github.com/ruby/thwait"
+  spec.license       = "BSD-2-Clause"
+
+  spec.files         = [".gitignore", "Gemfile", "LICENSE.txt", "README.md", "Rakefile", "bin/console", "bin/setup", "lib/thwait.rb", "lib/thwait/version.rb", "thwait.gemspec"]
+  spec.bindir        = "exe"
+  spec.executables   = spec.files.grep(%r{^exe/}) { |f| File.basename(f) }
+  spec.require_paths = ["lib"]
+
+  spec.add_development_dependency "bundler", "~> 1.16"
+  spec.add_development_dependency "rake", "~> 10.0"
+end
diff --git a/lib/thwait/version.rb b/lib/thwait/version.rb
new file mode 100644
index 0000000000..a3ffb3af16
--- /dev/null
+++ b/lib/thwait/version.rb
@@ -0,0 +1,3 @@
+class ThreadsWait
+  VERSION = "0.1.0"
+end
diff --git a/tool/sync_default_gems.rb b/tool/sync_default_gems.rb
index ee2e60ab51..b48a2ed7ed 100644
--- a/tool/sync_default_gems.rb
+++ b/tool/sync_default_gems.rb
@@ -32,6 +32,7 @@
 # * https://github.com/ruby/tracer
 # * https://github.com/ruby/shell
 # * https://github.com/ruby/forwardable
+# * https://github.com/ruby/thwait
 #
 
 $repositories = {
@@ -66,7 +67,8 @@ $repositories = {
   sync: 'ruby/sync',
   tracer: 'ruby/tracer',
   shell: 'ruby/shell',
-  forwardable: "ruby/forwardable"
+  forwardable: "ruby/forwardable",
+  thwait: "ruby/thwait"
 }
 
 def sync_default_gems(gem)
@@ -172,6 +174,10 @@ def sync_default_gems(gem)
     `cp -rf ../fcntl/ext/fcntl ext`
     `cp -f ../fcntl/fcntl.gemspec ext/fcntl`
     `git checkout ext/fcntl/depend`
+  when "thwait"
+    `rm -rf lib/thwait*`
+    `cp -rf ../thwait/lib/* lib`
+    `cp -rf ../thwait/thwait.gemspec lib`
   when "strscan"
     `rm -rf ext/strscan test/strscan`
     `cp -rf ../strscan/ext/strscan ext`