A 13-year-old boy was referred to orthopedic surgery for chronic intermittent pain and swelling of the left knee. Initial imaging was consistent with osteochondritis dissecans of the femoral condyle. Follow-up imaging demonstrated unexpected progression, with a mass extending into the notch, replacing the anterior cruciate ligament, and eroding the femoral and tibial condyles. Subsequent surgical biopsy and resection revealed tumoral calcinosis, with an ultimate diagnosis of autosomal recessive familial tumoral calcinosis. This case report highlights the radiographic appearance and progression of a rare disease in this unusual location and the differential diagnosis.