A 4-year-old, Spanish Mustang mare was evaluated because of a swelling on the dorsolateral aspect of its right carpus. The horse had no gait abnormalities. During radiographic examination of the carpus, a granular, radiopaque mass, approximately 3 cm in diameter, adjacent to the proximal row of carpal bones, was observed. The mineralised mass, which was located within the antebrachiocarpal joint, was removed through an arthrotomy with the horse anaesthetised. Histological features of the mass showed it to be overlain by partially denuded synovium, and included large deposits of mineral surrounded by multinucleated giant cells and separated by septa of mature fibrous connective tissue. These features were typical of calcinosis circumscripta. Swelling of the carpus resolved after the surgery, but the horse was lost to follow-up.