Streptococcus sanguis (S. sanguis) is a gram positive streptococcus bacteria which is found in the normal bacterial flora of the oral cavity and the upper respiratory tract. It has low virulence, but it can cause bacterial endocarditis through the blood circulation when dental calculus are removed from the teeth or during surgical treatment. Septic arthritis caused by S. sanguis has been reported as infecting the sternoclavicular joint and the knee joint, but it is a quite rare infectious disease that has not been reported in Korea. Therefore, the authors report a case of the septic arthritis in the knee joint caused by S. sanguis in a patient with osteoarthritis of the knee, who has the history of periodontitis.