A giant umbilical cord is a rare finding in mature newborns and originates from different developmental etiologies. We report on a case of a mature female newborn presenting a 50 × 8-cm giant umbilical cord without further malformations. Antenatal sonographic findings of a diffuse giant umbilical cord, elevated creatinine levels of 1.3 mg/dL in umbilical cord edema, gross and histopathological findings of allantoic remnants, and umbilical urinary discharge lead to the diagnosis of a patent urachus with retrograde micturition into the umbilical cord. Postnatal surgical repair was required. In antenatal sonography, cystic and diffuse changes should be considered in the differential diagnosis of a giant umbilical cord. In cases of diffuse enlargement, elevated umbilical creatinine can support the diagnosis of a patent urachus with open leakage into the Wharton's jelly. Appropriate surgical management is required.