Gonadal sex cord stromal tumors contain some of the most morphologically interesting neoplasms of the gonads, and these lead to many important issues in differential diagnosis. Granulosa cell tumors are much more common in females. Unclassified sex cord stromal tumor is rare. We report a case of a 22-year-old woman with acute ruptured unclassified sex cord stromal tumor of the right ovary during pregnancy. The biological behavior of unclassified sex cord stromal tumors resembles that of Sertoli-Leydig cell tumors of intermediate differentiation rather than poorly differentiated tumors, which may have been expected in view of the lack of specific differentiation. This finding is important with regard to postoperative management.