Tenosynovitis with psammomatous calcification (TPC) is an extremely rare condition. It was first described as a characteristic subtype of idiopathic calcifying tenosynovitis, with only 40 cases reported to date. Here, we present a case of TPC affecting a female patient in her late teens, with no relevant medical history. She presented with discomfort and pain in the right first toe. A 10-mm mushroom-like calcified mass was observed in the metatarsophalangeal joint on radiographs. The surgical specimen revealed chronic synovitis with calcification. Numerous psammomatous bodies are observed in the synovium, often with granulomatous reactions. After removal of the mass, no recurrence has been observed for 5 years. Although the etiology of TPC has been suggested to be related to repetitive trauma to the tendon or peritendinous soft tissue, the composition and mechanism of calcification remain unclear because of its rarity. In this report, we also discuss the calcification mechanism in TPC, supported by scanning electron microscopy and energy-dispersive x-ray spectroscopy findings.