Abstract γδ T cells play a critical role in early anti-tumor immunity and perform cytotoxicity via NKG2D for recognition and multiple cytotoxic factors for tumor killing. Recent studies have demonstrated pivotal roles of mTOR-mediated metabolism in the maturation, differentiation, and effector function of diverse immune cells, including DCs, NK cells, CD4+ T cell subsets, and CD8+ T cells, but the role of mTOR signaling in γδ T cells is barely known. Here, we showed that suppressing mTOR signaling in in vitro-expanded Vγ4 γδ T cells via the mechanistic inhibitor rapamycin enhanced their cytotoxicity against multiple tumor cell lines, and these cells performed better tumor-suppressing effects upon adoptive therapy. Further investigation revealed that elevated cytotoxicity was a result of up-regulation of NKG2D and TNF-α. Moreover, rapamycin treatment significantly decreased the expression of CISH and increased pSTAT5. The inhibition of STAT5 pathways via siRNA interference or a specific inhibitor eliminated the up-regulation of NKG2D and TNF-α in rapamycin-treated Vγ4 γδ T cells. These results uncovered an important role of mTOR signaling in the cytotoxic effector function of γδ T cells and provided a potential strategy to improve γδ T cell-based cancer immunotherapy.