Neonatal hydrocolpos is a rare condition that involves fluid accumulation in the vagina. On diagnostic imaging, the dilated vagina, along with the compressed uterus, can simulate a mature cystic teratoma with a mural nodule. Herein, we report the case of a newborn girl with congenital hydrocolpos that was caused by an imperforate hymen; the hydrocolpos mimicking a mature cystic teratoma on abdominal ultrasonography and magnetic resonance imaging. Any newborn girl with a pelvic cystic mass should be suspected as having a congenital vaginal obstruction manifesting as hydrocolpos or hydrometrocolpos. Thorough examination of the external genitalia, as well as imaging of the uterus and vagina, enables correct diagnosis and optimal treatment.