We present a case of adult onset Still's disease (AOSD) that was misdiagnosed as septic arthritis of the shoulder and knee. A forty-nine-year-old woman was admitted for pain in the left knee. The patient's medical history showed that she had undergone arthroscopic irrigation twice and an open debridement under the diagnosis of septic shoulder at another hospital. The laboratory and joint fluid analysis findings led us to suspect septic knee. Arthroscopic irrigation and antibiotics treatment were performed. At five weeks after discharge, she presented with pain in the same joint, fever, and rash. The symptoms were consistent with Yamaguchi's criteria for AOSD. We started corticosteroid therapy, and clinical remission was achieved. In conclusion, we suggest that AOSD should be considered as a diagnosis of exclusion to avoid misdiagnosis with septic arthritis.