It was found an unusual anatomical variation in a male cadaver that is consistent with the existence of an accessory muscle in the posterior area of the third distal of the forearm. Such structure arises in the posterior face of ulna and is fixed in the back of the proximal phalange of the middle finger. The described variation is also useful to the hand surgeon, as the anomalous muscle is an anatomical landmark for operations at this area. To our knowledge, this variation has not been cited in recent medical literature.