Spontaneous intraperitoneal extravasation of urine is rare. Urinary ascites occurs mostly in patients with the common primary obstructive lesion being posterior urethral valves. The second most common cause of urinary ascites is hydronephrosis due to congenital ureteropelvic junction obstruction. We report a case of urinary ascites with urinoma resulting from a ureteropelvic junction obstruction in an infant.