We describe an 18-year-old man with a renal transplant who developed septic metacarpophalangeal arthritis due to Mycobacterium gastri. He had several episodes of crystal induced synovitis, and treatment with intraarticular steroids was complicated 3 months later by iatrogenic septic arthritis. Appropriate treatment based on in vitro drug susceptibility was successful. This seems to be the first case of articular infection and the third report of human infection caused by this atypical mycobacteria.