Primary septic arthritis of the Acromio clavicular joint is an uncommon disorder and is rarely seen even in an immunocompromised individual. We report a case of primary septic arthritis of the acromio-clavicular (A-C) joint caused by Staphylococcus aureus without any predisposing factors. The patient was admitted with left shoulder pain, restricted movements and fever. Laboratory parameters showed high C-reactive protein, raised erythrocyte sedimentation rate and leucocytosis. Blood cultures were positive for Staphylococcus-aureus. Magnetic resonance imaging (MRI) using Gadolinium enhancement revealed marked effusion in the A-C joint. Aspiration from the A-C joint revealed a heavy growth of Staphylococcus-aureus. The patient was successfully treated with 8 weeks of appropriate antibiotics with complete resolution of infection and return to full function.