A key feature of serous ovarian cancer is frequent amplification in the 3q26 locus, which harbors the PRKCI gene. Here we show that PRKCI is amplified and overexpressed in a 78% of high-grade serous ovarian carcinoma. Our in vivo studies with orthotopic mouse models establish it as an ovarian cancer oncogene. This oncogenic property of PRKCI is mediated by regulation of YAP1 activity. Accordingly YAP1 knockdown partially rescues PRKCI mediated tumorigenesis. Integrated gene expression profiling and YAP1 promoter occupancy analyses reveal that PRKCI and YAP1 cooperate to transcriptionally regulate genes affecting the tumor immune microenvironment. Consistently, CD11b+Gr1+Ly6G+ MDSCs are increased in the tumor microenvironment of PRKCI over-expressing tumors. Importantly, we show that elevated PRKCI expression in high-grade serous ovarian carcinomas strongly correlated with assignment to immunoreactive subtype. In summary, we identify PRKCI and YAP1 as key mediators of a tumor-promoting immunosuppressive microenvironment in ovarian cancers.