INTRODUCTION: The human hand is a complex anatomic entity consisting of many muscles, nerves, and vessels, thus providing a special ability to perform accurate and meticulous movements. In this group of muscles are the four dorsal interosseous muscles.CASE PRESENTATION: A distinct supernumerary fourth dorsal interosseous muscle was found in the right hand of the cadaver of a 76-year-old Caucasian man without any other concomitant abnormality.CONCLUSIONS: The presence of such an additional muscle in the hand should be considered in the management of hand deformities, whether the treatment is conservative or surgical.