Purpose <p>The extensor indicis muscle is an important muscle of the deep layer of the posterior compartment of the forearm, as it allows independent extension of the index finger. Numerous forms of variations of the extensor indicis muscle have been reported. This case report aims to describe a rare unilateral presence of an accessory muscle belly of the extensor indicis.</p> Case report <p>An incidental finding during the routine dissections of the posterior compartments of the forearm and the dorsum of the hand is presented. The spindle-shaped accessory muscle of the extensor indicis originated from the distal third of the ulna and inserted onto the tendon of the normal extensor indicis muscle just before entering the fourth osseofibrous tunnel of the extensor retinaculum of the wrist.</p> Conclusion <p>This case report shows a rare variation pattern of the extensor indicis muscle; an accessory head/belly that originates in the distal ulna and insert onto the main tendon of the extensor indicis at the level of the wrist joint. Knowledge of the variations of the extensor indicis muscle may be important for clinicians for the appropriate diagnosis and management of wrist and hand conditions.</p>