Extensor carpi ulnaris is a muscle, located in the forearm of humans that acts to extend and adduct the wrist.

Being an extensor muscle, extensor carpi ulnaris is on the posterior side of the forearm. It attaches to the lateral epicondyle of the humerus, though it crosses the forearm to the ulnar (medial) side, and attaches to the base of the 5th metacarpal.

It is innervated by a branch of the radial nerve, namely the posterior interosseus nerve. Its arterial supply comes from the ulnar artery.