Urachal diverticulum is an infrequent finding and its perforation as a cause of fetal urinary ascites has not been reported before. This is a case report on an infant with antenatal diagnosis of fetal ascites. This infant was delivered via cesarean section and needed mechanical ventilation owing to a massive ascites that required paracentesis. She did not void for 20 h until a urethral catheter was placed. The biochemical analysis of the ascitic fluid compared with the serum and urine was suggestive of urine ascites. The voiding cystourethrogram (VCUG) showed a leakage at the remnant of urachal diverticulum. The urethral catheter was maintained for 3 weeks until the repeated VCUG confirmed a sealed-off urachal diverticulum. With removal of the catheter, the infant was able to void spontaneously and was discharged home on continued prophylactic antibiotics. As symptomatic urachal remnants have an increased potential for malignant transformation, a close follow-up by a urologist was recommended.