Background: Ovarian steroid cell tumors (SCTs) are rare and usually benign, although 25-43% are reportedly malignant. The cytologic findings of these rare ovarian tumors have almost never been reported. Case: We report a rare case of a malignant ovarian SCT with peritoneal dissemination and malignant ascites in a 40-year-old woman. Her tumor was classified as stage IIB (pT2bNoM0) according to the FIGO (International Federation of Gynecology and Obstetrics) classification system, and she was treated with adjuvant chemotherapy following staging laparotomy. Cytology of the ascitic fluid revealed large, polygonal-to-round cells and multinucleated cells with atypia, appearing in clusters with slight overlapping or as isolated tumor cells. Numerous tumor cells had small central round or eccentric nuclei with conspicuous nucleoli, and a moderate-to-abundant amount of cytoplasm, varying from granular and eosinophilic to pale and multivacuolated (foamy), with cannibalism formations. The nuclear chromatin was fine and granular, with irregular distribution and nuclear-membrane thickening. Conclusion: These may be the first reported cytology results for ascites with a malignant SCT. Our patient's cytological ascitic findings, rather than the histopathologic features of the original and disseminated tumors, represent the malignant features of the tumor.