This report describes a case of giant cystic meconium peritonitis (GCMP) associated with a cloacal anomaly. Antenatal ultrasonography and magnetic resonance imaging demonstrated persistent fetal ascites, bilateral hydronephrosis, and 3 pelvic cystic structures. The baby girl showed duplicated hydrocolpos and a single orifice of the cloaca with a long common channel inducing a urinary outflow obstruction. After constructing a diversion colostomy, a cutaneous vesicostomy was necessary to prevent recurrent urinary tract infections. These findings are consistent with a prenatal diagnosis of cloacal anomalies, thus suggesting an association with severe obstruction of lower urinary tract and meconium peritonitis. Most of reported cases of meconium peritonitis associated with the cloaca show fibroadhesive types with scattered intraperitoneal calcifications and adhesions. However, the present case showed a rare GCMP suggesting continuous urinary influx via the fallopian tubes until the later stage of intrauterine life.