Abstract Sertoli–Leydig cell tumor (SLCT) is a rare tumor involving the ovary. Approximately 20% of SLCT are associated with heterologous elements that are either of endodermal or mesodermal origin. The gastrointestinal-type epithelium is the most commonly described endodermal heterologous element. SLCT with benign and borderline mucinous neoplasm has been reported in the literature. However, SLCT with mucinous adenocarcinoma as heterologous element has been rarely documented. Herein, we describe a rare case of intermediately differentiated Sertoli–Leydig cell tumor with mucinous adenocarcinoma as the heterologous element in a 21-year-old woman. She presented with throbbing lower abdominal pain and was found to have a large, complex left ovarian mass on imaging studies. She underwent left salpingo-oophorectomy, appendectomy and lymph node staging. Gross examination of the surgical specimen showed a large, encapsulated, solid-cystic mass completely replacing the ovary. Microscopically, the tumor was composed of intermediately differentiated Sertoli–Leydig cell tumor and well-differentiated mucinous adenocarcinoma. Interestingly, the bulk of the tumor (more than 90%) was composed of mucinous adenocarcinoma, whereas the SLCT component comprised less than 10% of the total tumor. The mucinous adenocarcinoma expressed positivity for CK20, CEA, CDX2 and CK7, and the SLCT component was positive for inhibin expression. The histopathological features and results of immunostaining were consistent with the diagnosis of the intermediately differentiated SLCT with mucinous adenocarcinoma as the heterologous element. This case was a diagnostic challenge as more than 90% of the tumor was composed of mucinous adenocarcinoma and SLCT constituted only the minor part of the tumor. This feature was in contrast to the previously described two cases, where mucinous adenocarcinoma as heterologous element was present as microscopic foci. This case highlights the importance of identifying the SLCT component in a case of an apparently pure mucinous adenocarcinoma in a young patient.