To report a case septic arthritis with a rare pathogen in a immunosuppressed child.Male patient, 6 years old, had liver transplant 5 and half years ago due to biliary atresia. Patient was using tacrolimus 1mg q.12hours. This patient started to have pain in left foot and ankle and had one episode of fever 3 days before hospital admission. Physical Examination showed weight 17kg, height 109cm, temperature 36,4°C, with pain, swelling and heat in the left ankle, without other clinical signs. Initial tests: hemoglobin 11,7g/dL hematocrit 36.4%, leukocyte count 17600/uL (7% banded neutrophils, 70% segmented neutrophils, 2% eosinophils, basophils 1%, 13% lymphocytes, 7% monocytes) C-reactive protein 170,88mg/L. Joint ultrasound showed moderate effusion in the site. Patient was submitted to surgical procedure and S. multivorum was isolated from the effusion. The germ was susceptible to broad spectrum cephalosporins (ceftriaxone and cefepime) and fluoroquinolones (ciprofloxacin and levofloxacin), and it was resistant to carbapenemic antibiotics and aminoglycosides. He was treated intravenously with oxacillin for 15 days and ceftriaxone for 13 days, and orally with ciprofloxacin for 15 days, with good outcome.The Sphingobacterium multivorum is a gram negative bacillus that belongs to Flavobacteriaceae family and it is considered non-pathogenic. It has rarely been described as a cause of infections in humans, especially in hospital environment and in immunosuppressed patients. This case report is relevant for its unusual etiology and for the site affected, which may be the first case of septic arthritis described.