Abstract Natural killer (NK) cells play a crucial role in both innate immunity and the activation of adaptive immunity. The activating effect of Mn2+ on cyclic GMP‐AMP(cGAS)–stimulator of interferon genes (STING signaling has been well known, but its effect on NK cells remains elusive. In this study, we identified the vital role of manganese (Mn2+) in NK cell activation. Mn2+ directly boosts cytotoxicity of NK cells and promotes the cytokine secretion by NK cells, thereby activating CD8+ T cells and enhancing their antitumor activity. Furthermore, Mn2+ can simultaneously activate NK‐cell intrinsic cGAS and STING and consequently augment the expression of ubiquitously transcribed tetratricopeptide repeat on chromosome X (UTX to promote the responsiveness of NK cells. Our results contribute to a broader comprehension of how cGAS–STING regulates NK cells. As a potent agonist of cGAS–STING, Mn2+ provides a promising option for NK cell‐based immunotherapy of cancers.