We present an unusual case of a congenital left subcostal hernia in a female born at 39 weeks by caesarean delivery to a 39 year-old gravida(2)para(1-2)abortus(0) mother. The pregnancy was complicated by gestational diabetes, although she had normal prenatal imaging. After an uneventful caesarean delivery, the patient's initial physical examination was normal although subsequently was found to have a left subcostal hernia. During surgical evaluation, a reducible hernia was found along with left renal agenesis and no underlying masses. At 4 weeks of age, the patient's hernia was successfully repaired with no sequela. We present the first reported case of a congenital subcostal hernia, evaluation, and treatment.