Interleukin-15 (IL-15) controls both the homeostasis and the peripheral activation of Natural Killer (NK) cells. The molecular basis for this duality of action remains unknown. Here we report that the metabolic checkpoint kinase mTOR is activated and boosts bioenergetic metabolism upon NK cell exposure to high concentrations of IL-15 whereas low doses of IL-15 only triggers the phosphorylation of the transcription factor STAT5. mTOR stimulates NK cell growth and nutrient uptake and positively feeds back onto the IL-15 receptor. This process is essential to sustain NK cell proliferation during development and acquisition of cytolytic potential upon inflammation or virus infection. The mTORC1 inhibitor rapamycin inhibits NK cell cytotoxicity both in mouse and human, which likely contribute to the immunosuppressant activities of this drug in different clinical settings.