Septic arthritis may represent a direct invasion of joint space by various microorganisms, including bacteria, viruses and fungi. Although any infectious agent may cause bacterial arthritis, bacterial pathogens are the most significant because of their rapidly destructive nature. We present a case of septic arthritis in a 56-year old male patient due to Streptococcus viridans which is member of the viridans group streptococci. Patient was admitted to Our Hospital presented as fever of unknown origin, losing more than 30 kg of body weight during couple of months, and anemia of chronic disease as paraneoplastic process. He had long history of arterial hypertension and stroke. There was swelling and pain of the right sternoclavicular joint and precordial systolic murmur in physical status. A large diagnostic panel has been made, computerized tomography (CT) of right sternoclavicular joint showed widening of periarticular soft tissue and loss of clavicular corticalis. Cytologic analysis of synovial fluid showed more than 90% of polymorphonuclear leukocytes. There were no crystals on microscopic examination and Gram stain of fluid was negative. Blood cultures were positive for S. sanguis and there was a consideration about possible periodontal disease. Stomatologic examination verified periapical ostitis and extraction of potential cause of infection has been done. Therapy with benzilpenicilline was followed by the gradual improvement of clinical and laboratory parameters. Although viridans group streptococci and Streptococcus sanguis in particular are rare causes of septic arthritis in native joints, they should be considered in the differential diagnosis of periodontal disease.