Urachal carcinomas are rare tumors, and the majority of them are adenocarcinomas. Up to now, only 21 urachal urothelial carcinomas (UCas) have been reported. Here, we describe a case of high grade UCa arising from the urachal remnants.A 66-year-old man presented with voiding difficulties. Prostate specific antigen (PSA) was 5.46 ng/mL. Prostatic adenocarcinoma (PCa) (Gleason score 6) was diagnosed by needle biopsies. After the diagnosis of high grade, muscle invasive UCa with intact mucosa on frozen examination of the dome of bladder wall during the radical prostatectomy operation (RPO), partial cystectomy was performed. Microscopically, among the tumoral islands, cystic structures lined by cells with a benign appearance, which are considered urachal remnants, were noted. Cytokeratin 7 and high-molecular-weight cytokeratin were strongly positive, PSA and carcinoembryonic antigen were negative. Radiotherapy was given for both UCa and PCa. Six months after the diagnosis, an undifferentiated tumor was detected in a bladder transurethral resection specimen; thus, chemotherapy was given. After 1 course of chemotherapy, the patient was doing well.The origin of urachal carcinomas is usually obscured as it is a highly invasive carcinoma. The patient presented here was diagnosed incidentally during RPO. Although the stage was advanced, the tumor was detected before urachal remnants were destroyed. The treatment choice for urachal carcinomas is cystectomy. Adjuvant chemotherapy and radiotherapy are controversial.