The PI3K/Akt/mTOR axis in ovarian cancer is frequently activated and implicated in tumorigenesis. Specific targeting of this pathway is therefore an attractive therapeutic approach for ovarian cancer. However, ovarian cancer cells are resistant to PP242, a dual inhibitor of mTORC1 and mTORC2. Interestingly, blockage of GLS1 with a selective inhibitor, CB839, or siRNA dramatically sensitized the PP242-induced cell death, as evident from increased PARP cleavage. The anti-cancer activity of CB-839 and PP242 was abrogated by the addition of the TCA cycle product alpha-ketoglutarate, indicating the critical function of GLS1 in ovarian cancer cell survival. Finally, glutaminolysis inhibition activated apoptosis and synergistically sensitized ovarian cancer cells to priming with the mTOR inhibitor PP242. GLS1 inhibition significantly reduced phosphorylated STAT3 expression in ovarian cancer cells. These findings show that targeting glutamine addiction via GLS1 inhibition offers a potential novel therapeutic strategy to overcome resistance to PI3K/Akt/mTOR inhibition.