Lateral malleolar bursitis rarely progresses to septic arthritis. In our case, the 27 year old man visited due to progressive left ankle pain, despite the antibiotics treatment of lateral malleolar bursitis. 8 years ago, modified Brostrom procedure was performed owing to chronic ankle instability. Previous surgery altered anatomical structure of lateral ankle bursa, so it may cause the infection to spread to the ankle joint. We reported rare case of secondary septic arthritis caused by lateral malleolar bursitis.