BACKGROUND We present a case of Streptococcus pneumoniae polyarticular septic arthritis in a patient with rheumatoid arthritis receiving a single infusion of infliximab. METHODS A 38-year-old Japanese man with a 5-year history of seronegative rheumatoid arthritis had previously received sulphasalazine and methotrexate therapies and was on regular low-dose prednisolone therapy. Despite these treatments, his disease activity remained high and infliximab was introduced in addition to methotrexate, prednisolone, and folic acid. However, he was admitted to hospital with a fever of 40.6°C, chills, and polyarthralgia eight days after the first infusion of infliximab. His joints were swollen, painful, and warm. Laboratory data showed marked acute inflammation. He was diagnosed with bacterial septic polyarthritis, and emergency surgical joint lavage and drainage was performed at the knees along with needle aspiration and lavage of the ankles and right wrist. He was then given intravenous antibiotic therapy for 31 days. He made a good recovery and was discharged on day 37. CONCLUSIONS We believe this is the first reported case of severe pneumococcal septic arthritis requiring hospitalization in a patient treated with infliximab. S. pneumonia is now a well-recognized but uncommon cause of polyarticular septic arthritis that can lead to cessation of therapy, as in our patient's case.