Rationale:Malignant mesenchymoma (MM) is defined as a heterogeneous malignant soft tissue tumor that consists of 2 or more distinctly different mesenchymal components in addition to fibrosarcomatous elements. Bladder MM was rarely reported in the literature and there are only 5 cases of primary bladder MM documented in English literature to date.Patient concerns:A 58-year-old male complained of difficulty in urination and intermittent gross hematuria for 3 months. Doppler ultrasound scan revealed an avascular and homogeneous hypoechoic mass measured 6.5 × 9 cm in the bladder. Computed tomography showed a homogeneous solid mass in the bladder.Diagnoses:Pathology revealed spindle-shaped tumor and proliferation of poorly differentiated immature mesenchymal cells rich in eosinophilic cytoplasm with hyperchromatic sticklike nuclei. Immunohistochemical examinations were positive for CD117.Interventions:The patient was diagnosed with presence of bladder tumor and underwent radical cystectomy; the optimal treatment strategy was reviewed and discussed.Outcomes:There was no recurrence or metastasis during a 16-month follow-up.Lessons:Our case study demonstrated bladder MM with a relatively indolent clinical course. A multidisciplinary approach including surgery, radiotherapy, and chemotherapy may be useful. Radical resection is the most important determinant of clinical outcome. Generally, the clinical outcome and prognosis of mesenchymoma are favorable.