This report describes a 9-year-old girl with pigmented villonodular synovitis of the right knee. She presented with a chronic bloody effusion of the knee. The diagnosis of pigmented villonodular synovitis was suggested by the findings on magnetic resonance imaging and confirmed at the time of arthroscopic synovectomy. This report emphasizes the importance of considering pigmented villonodular synovitis in the differential diagnosis of chronic hemarthrosis in children.