Pigmented villonodular synovitis is a benign proliferative tumor of the synovium. It is very rare, and most cases occur in the knee joint. In this article, we report a case of localized pigmented villonodular synovitis in the knee joint of a 14- yearold boy. This condition is rare in the knees of the children. We preferred to remove the tumor with arthrotomy instead of arthroscopy for two reasons: (i) the patient was obese, (ii) we thought that recurrence risk was high after arthrotomy.