Sesamoiditis involving the hand is uncommon, usually reported in the thumb, and has not been reported in the index finger. As rare as this clinical entity remains, its presentation simulating an acute suppurative flexor tenosynovitis is even more rare. We report a patient who presented with Kanavel's 4 cardinal signs of acute suppurative tenosynovitis who was subsequently found to have an acute sesamoiditis of the index finger. This finding was supplemented by a cadaveric and radiographic study to better delineate the anatomy of the index sesamoid and further explain the clinical presentation.