Human NK cells display extensive phenotypic and functional heterogeneity among healthy individuals, but the mechanism responsible for this variation is still largely unknown. Here, we show that a novel immune receptor, T-cell immunoglobulin and ITIM domain (TIGIT), is expressed preferentially on human NK cells but shows wide variation in its expression levels among healthy individuals. We found that the TIGIT expression level is related to the phenotypic and functional heterogeneity of NK cells, and that NK cells from healthy individuals can be divided into three categories according to TIGIT expression. NK cells with low levels of TIGIT expression show higher cytokine secretion capability, degranulation activity, and cytotoxic potential than NK cells with high levels of TIGIT expression. Blockade of the TIGIT pathway significantly increased NK-cell function, particularly in NK cells with high levels of TIGIT expression. We further observed that the TIGIT expression level was inversely correlated with the IFN- secretion capability of NK cells in patients with cancers and autoimmune diseases. Importantly, we propose a novel mechanism that links TIGIT expression with NK-cell functional heterogeneity, and this mechanism might partially explain why individuals have different susceptibilities to infection, autoimmune disease, and cancer.