BACKGROUND In this single case report study, we describe an anatomical variance of a brachioradialis muscle with two muscle bellies and two tendons with their common insertion on the suprastyloid crest of a distal radius. CASE SUMMARY After a self-mutilating forearm injury of a borderline personality patient, we observed the anatomical variation of brachioradialis tendon. After tendon repair and aftercare the muscle function recovered completely. Five months after surgery the forearm magnetic resonance imaging was done revealing a supernumerary brachioradialis muscle. CONCLUSION Anatomical variations of forearm muscles and tendons are not rare. The treating surgeons should be aware of their existence and anatomical relationships.