Background: Ureaplasma parvum is usually part of the normal genital flora. Rarely can it cause invasive infections such as genitourinary infections, septic arthritis, or meningitis. Case presentation: Here we present the first description of chronic ureterocystitis in a 56-year-old immunocompromised patient, complicated first by reactive arthritis and secondarily by contralateral septic arthritis due to U. parvum infection. U. parvum was detected in synovial fluid and in a urine sample. Treatment consisted of double-J stenting and targeted antibiotic therapy. Evolution showed resolution of urinary symptoms and clinical improvement of arthritis despite functional sequelae. Conclusions: Given the high prevalence of U. parvum colonisation, this diagnosis should remain a diagnosis of exclusion. However, because of the difficulty in detecting this microorganism, it should be considered in unexplained subacute urethritis or arthritis, including reactive arthritis, especially in immunosuppressed patients. Real-time PCR positivity in the absence of a differential diagnosis should not be overlooked.