Primary Leiomyosarcoma of the seminal vesicle is a very rare condition. We report a case of a 74-year-old man with a tumour detected by rectal symptoms with pelvic pain and dysuria at ultrasonography. Computed tomography and magnetic resonance imaging suggest an origin in the left seminal vesicle and did not show a clear cleavage plan with the rectum and a right hydroureteronephrosis was also present. A radical vesiculo-cystoprostatectomy with ileal conduit and bilateral pelvic lymphadenectomy was performed, a sigmoidectomy with end colostomy was performed also. Pathological examination showed a high grade (G3) leiomyosarcoma of the seminal vesicle.