In this case report a rare case of Ureaplasma parvum arthritis affecting the left knee, hip, and the sacroiliac joint in a 57-year-old woman is investigated. The patient was initially treated with intravenous piperacillin/tazobactam and later on meropenem with no clinical effect. After surgery of the hip and knee, tissue biopsies and synovial fluids were all culture negative. Eventually, Ureaplasma parvum was detected by 16S analysis, and the antibiotic regimen was changed to intravenous moxifloxacin. The patient improved and after four weeks the therapy was changed to lifelong oral therapy with moxifloxacin.