Sertoli-Leydig cell tumors (SLCT) are rare tumors of the ovary with a peak incidence in the second to third decade of life. Serous borderline tumors (SBT) are epithelial ovarian neoplasms which occur at a median age of 50 years. A co-occurrence of SLCT and SBT has not yet been reported. Here, we describe a case of a 16-year-old girl who presented with irregular menses, virilization, and an abdominopelvic mass. The mass was surgically removed and an intraoperative consultation revealed an 18.5 cm solid and cystic ovarian mass with the presence of co-existing SLCT and SBT. The diagnosis was confirmed on permanent sections after extensive sampling and immunohistochemical stains. The SLCT showed positive staining for calretinin, inhibin, CD99, and androgen receptor. MART-1 immunostain highlighted the Leydig cells. The SBT showed classic features including hierarchically branching papillae lined by stratified serous epithelium. This pediatric case is the first reported case of a Sertoli-Leydig cell tumor arising in association with a serous borderline tumor.