Sclerosing stromal tumors of the ovary are very rare. We report the case of a 29-year-old pregnant woman who presented with a history of preterm labor and gestational hypertension. A huge mass measuring 30 cm in greatest dimension was recognized on the right ovary during cesarean section. Signet ring cell infiltration was seen during intraoperative frozen section consultation. She underwent a right salpingo-oophorectomy, and the permanent diagnosis was sclerosing stromal tumor infiltrated by signet ring cells. Pregnancy-related changes were seen in tumor morphology. Signet ring cells were immunoreactive with antibodies MUC5ac, MUC2, and EMA. Thus, stromal cells were immunoreactive with antibodies inhibin and calretinin. Endoscopic biopsy of the stomach showed a signet ring cell carcinoma, from which the metastasis had originated. This is the first reported case in the literature, and is notable for its challenging differential diagnosis, which included a pregnancy luteoma and primary signet ring stromal tumor of the ovary.