This case report highlights an unusual cause of anterolateral knee pain. The popliteus muscle arises from three origins--that is, the lateral femoral condyle, the fibula head, and the lateral meniscus--and inserts into the proximal tibia above the soleal line. It may be subjected to a number of pathologies including tenosynovitis, acute calcific tendonitis, rupture, and even avulsion. In this case, the diagnosis of popliteus tendon tenosynovitis was not made from magnetic resonance imaging findings, but was confirmed and successfully treated during arthroscopic examination.