Septic arthritis of the acromioclavicular (AC) joint is a rare but rapidly destructive etiology of acute shoulder pain. We report a case of septic AC joint in the absence of trauma or intravenous drug use. A diabetic man presented with severe right shoulder pain and was later diagnosed with septic arthritis of the AC joint by magnetic resonance imaging (MRI) and positive blood cultures. Chronic diabetic foot ulcers were the source of the patient's group B streptococcus bacteremia.