A 7-year-old boy was referred for evaluation of precocious puberty, evidenced by penile enlargement and pubic hair formation. His testicular size was prepubertal bilaterally. A comprehensive hormonal evaluation showed an elevated serum testosterone value (4.0 nmol/L) and a prepubertal gonadotropin value. A 0.9-cm heterogenous left testicular mass was detected on scrotal ultrasonography. Inguinal exploration was performed with ultrasound-guided open testicular biopsy and orchiectomy. Pathologic evaluation of the orchiectomy specimen showed the unclassified type of a mixed germ cell sex cord stromal tumor (MGCSCST), composed of neoplastic Sertoli cells and seminoma-like germ cells. Isolated previous reports of unclassified MGCSCSTs of the testis are now thought to be reports of sex cord stromal tumors with entrapped non-neoplastic germ cells. In our patient, the germ cells appeared to be neoplastic with aberrant expression of c-kit and placental alkaline phosphatase, a high proliferative rate, and DNA aneuploidy. Postoperatively, the patient's serum testosterone concentrations returned to prepubertal values (<0.2 nmol/L) and puberty was halted. This case represents a novel cause of precocious puberty.