diff --git a/core/mutex.rbs b/core/mutex.rbs
new file mode 100644
index 0000000..4040225
--- /dev/null
+++ b/core/mutex.rbs
@@ -0,0 +1,13 @@
+class Mutex
+  def initialize: () -> void
+
+  def lock: () -> self
+
+  def unlock: () -> self
+
+  def try_lock: () -> bool
+
+  def locked?: () -> bool
+
+  def owned?: () -> bool
+end
diff --git a/test/mutex.rb b/test/mutex.rb
new file mode 100644
index 0000000..9ca5d54
--- /dev/null
+++ b/test/mutex.rb
@@ -0,0 +1,13 @@
+# frozen_string_literal: true
+
+mutex = Mutex.new
+
+mutex.lock
+
+mutex.unlock
+
+mutex.try_lock
+
+mutex.locked?
+
+mutex.owned?