Urachal mucinous cystadenocarcinoma causing pseudomyxoma peritonei is very rare. We report a case of a 59-year-old man with urachal mucinous adenocarcinoma associated with pseudomyxoma peritonei, and our radiologic findings. Ultrasonography revealed a well delineated, large cystic tumor adjacent to the anterior wall of the abdomen. Computed tomography and magnetic resonance imaging revealed a tumor of which the left posterior wall was defective. A large amount of ascites was present in the peritoneal cavity. The ascites caused displacement of the intestinal structures toward the dorsal region. The tumor wall and septa in the ascites were well enhanced on contrast-enhanced images. Radiologically, pseudomyxoma peritonei due to rupture of urachal cystic tumor was considered. The pathologic diagnosis was mucinous adenocarcinoma and pseudomyxoma peritonei.