Tumors arising in an undescended testis are rare in infants. We report a mature teratoma in an intraabdominal testis of a 2-month-old boy. He presented with a large left-sided abdominal mass and nonpalpable left testis. Abdominal ultrasound and computed tomography showed a well-defined, sizable cystic tumor with focal calcification in the lower abdomen. Tumor markers were all within normal limits. Surgical resection revealed a large tumor arising from an intraabdominal left testis, and histologic analysis identified a mature teratoma. The tumor was completely removed, and there was no recurrence at follow-up 3 years later. The histologic condition of prepubertal intraabdominal testicular (IAT) tumors is quite different from that of testicular tumors in childhood descended testes and postpubertal IAT tumors. A total of 30 cases of IAT tumors in children, including our case, were reviewed to elucidate this issue.