A case of infected urachal cyst in an adult presenting as an erythematous suprapubic mass was demonstrated by the intravenous urography (IVU), ultrasonography (US) and computed tomography (CT). This report illustrated the value of complete radiologic evaluation in establishing the diagnosis and surgical planning of urachal cyst. IVU disclosed upward tenting of the bladder dome. US showed a cystic mass between bladder dome and anterior abdominal wall. CT gave the best delineation of the anatomical location of the cyst with sagittal reconstruction and intravenous contrast enhancement.