BACKGROUND: Progesterone rapidly induces ovarian cancer cell death through non-genomic actions mediated by the membrane progesterone receptor (mPR). AIMS: We investigated the combined effects of progesterone and SN38, an active metabolite of irinotecan, on ovarian cancer cells. METHODS AND RESULTS: mPR-positive and PR-negative ovarian cancer cell lines were utilized in experiments. Tumor cells were exposed to SN38 or cisplatin for 48 h following exposure to progesterone for 30 min. The viable cell counts were measured using a colorimetric assay and the expression of topoisomerase I (TOPO-I), the direct target of SN38, was observed with or without exposure to progesterone. Moreover, we investigated the relationship between several types of programmed cell death and the SN38 sensitivity enhancement effect of progesterone using specific cell death inhibitors. The chemosensitivity to SN38 was 8.7- to 26.0-fold higher with the administration of progesterone than that without (p < 0.01), but not to cisplatin in ovarian cancer cells. Progesterone suppressed the expression of TOPO-I mRNA by less than 50% (p < 0.01). Furthermore, among various programmed cell death inhibitors, only the ferroptosis inhibitor attenuated the progesterone-induced SN38 chemosensitivity enhancement effect. CONCLUSIONS: Progesterone increased sensitivity to SN38 by suppressing TOPO-I expression and inducing ferroptosis. The combination of progesterone and irinotecan could be a novel treatment modality for ovarian cancer.