Pes anserinus pain syndrome is a common, clinically defined condition that is characterized by pain around the medial knee and tenderness over the upper medial tibia. The anserine bursa could be the site of proliferative and inflammatory conditions due to knee osteoarthritis, leading to pain and fluid retention. However, rupture of the pes anserinus is rare. Herein, we present a case of rupture of the pes anserine bursa in a patient with pes anserine pain syndrome and osteoarthritis. Physicians should consider rupture of the pes anserine bursa as a differential diagnosis of acute unilateral lower leg swelling. J. Med. Invest. 66 : 211-212, February, 2019.