Epithelial ovarian cancer (EOC) is the fifth leading cause of cancer deaths among women in the United States, but its pathogenesis is poorly understood1,2,3. Some epithelial cancers are known to occur in transitional zones between two types of epithelium, whereas others have been shown to originate in epithelial tissue stem cells4,5,6. The stem cell niche of the ovarian surface epithelium (OSE), which is ruptured and regenerates during ovulation, has not yet been defined unequivocally. Here we identify the hilum region of the mouse ovary, the transitional (or junction) area between the OSE, mesothelium and tubal (oviductal) epithelium, as a previously unrecognized stem cell niche of the OSE. We find that cells of the hilum OSE are cycling slowly and express stem and/or progenitor cell markers ALDH1, LGR5, LEF1, CD133 and CK6B. These cells display long-term stem cell propertiesex vivoandin vivo, as shown by our serial sphere generation and long-term lineage-tracing assays. Importantly, the hilum cells show increased transformation potential after inactivation of tumour suppressor genesTrp53andRb1,whose pathways are altered frequently in the most aggressive and common type of human EOC, high-grade serous adenocarcinoma7,8. Our study supports experimentally the idea that susceptibility of transitional zones to malignant transformation may be explained by the presence of stem cell niches in those areas. Identification of a stem cell niche for the OSE may have important implications for understanding EOC pathogenesis.