Mycobacterium marinum is an atypical mycobacterium found in salt and fresh water. M. marinum infection occurs following skin trauma in fresh or salt water and usually presents as a localized granuloma or sporotrichotic lymphangitis. It rarely affects the musculoskeletal system. We describe a patient who presented with subcutaneous nodules and an inflammatory arthritis that was thought to be rheumatoid arthritis, and was treated as such with corticosteroids, methotrexate, and anti-tumor necrosis factor-alpha therapy, with worsening of his arthritis.