This document describes the case of a female infant born in 2015 who presented with decreased appetite and abdominal distension. Imaging showed a large retroperitoneal tumor compressing her kidneys. She received neoadjuvant chemotherapy but the tumor did not decrease significantly in size. She then underwent surgery to excise the large tumor, which pathology later confirmed was an immature teratoma. The key points are that retroperitoneal teratomas can present in infancy, often with abdominal distension, and complete surgical excision is needed for cure while avoiding injuries to major blood vessels given the tumors' large size and potential for vascular involvement.