A key feature of ovarian high-grade serous carcinoma is frequent amplification of the 3q26 locus. Here we show that PRKCI, located on the 3q26 locus is not only amplified and overexpressed in 78% of HGSC patient samples but is also expressed in early fallopian tube lesions, called Serous Tubal Intraepithelial Carcinoma. In vivo studies in a transgenic mouse model establish PRKCI as an ovarian cancer oncogene and identify YAP1 as a downstream regulator. Together, PRKCI and YAP1 regulate TNF to promote an immune suppressive microenvironment and inhibit cytotoxic T-cell infiltration in tumors. High PRKCI expressing human ovarian tumors show decreased cytotoxic T-cell infiltration. Taken together, we identify PRKCI and YAP1 as key mediators of a tumor-promoting immune microenvironment in ovarian cancer.