Flexor carpi ulnaris (FCU) is an ever-present muscle of the anterior flexor compartment of the forearm. Variations of FCU are uncommon, with additional slips or heads of muscles described, and only one reported case of an accessory muscle. We describe a unique clinical case report in which an accessory FCU was identified and describe the findings of 5,000 cadaveric dissections of the forearm, performed as part of an ongoing institutional study of anatomical variations. An aberrant accessory forearm flexor muscle was identified incidentally at the wrist during surgery for an anterior interosseous to ulnar nerve transfer for management of ulnar nerve palsy. This muscle was seen running superficial to the ulnar nerve and radial to the FCU proper, arising from the common flexor origin and inserting at the triquetral carpal bone. This was therefore suitably acknowledged as an accessory FCU . The anomaly was identified as bilateral using ultrasound imaging, and was found to be anomalously innervated by the median nerve with nerve conduction studies. A subsequent review of 5,000 cadaveric dissections of the forearm did not identify any such variations related to FCU, despite identifying a range of variations of the other forearm flexor musculature. While the scarcity of this anomaly is thus highlighted, consideration of an accessory FCU, and its aberrant innervation is important in a range of surgical approaches.