A 22-year-old man was referred to our hospital with macroscopic hematuria and consistent anal pain. Magnetic resonance imaging revealed an enlarged prostate tumour invading the bladder and rectum. A biopsy revealed an unclassified spindle cell sarcoma. Subsequently, radical cystoprostatectomy and resection of the rectum were performed. A histopathological examination revealed a prostatic malignant phyllodes tumour with a negative surgical margin. However, a local recurrence was identified 2 months after surgery. Induction therapy included 4 cycles of systemic chemotherapy comprising etoposide with ifosfamide and cisplatin. Although a partial response was observed at the local site, lung metastasis developed. Second-line chemotherapy with ifosfamide and doxorubicin with radiotherapy to the pelvis was administered and led to complete regression; however, its efficacy was transient. Although additional chemotherapy was administered, the patient eventually died due to the rapidly growing, recurrent tumour.