The process of affinity maturation, whereby T and B cells bearing antigen receptors with optimal affinity to the relevant antigen undergo preferential expansion, is a key feature of adaptive immunity. Natural killer (NK) cells are innate lymphocytes capable of “adaptive” responses after cytomegalovirus (CMV) infection. However, whether NK cells are similarly selected on the basis of their avidity for cognate ligand is unknown. Here, we showed that NK cells with the highest avidity for the mouse CMV glycoprotein m157 were preferentially selected to expand and comprise the memory NK cell pool, whereas low-avidity NK cells possessed greater capacity for interferon-γ (IFN-γ) production. Moreover, we provide evidence for avidity selection occurring in human NK cells during human CMV infection. These results delineate how heterogeneity in NK cell avidity diversifies NK cell effector function during antiviral immunity, and how avidity selection might serve to produce the most potent memory NK cells. Display omitted •Ly49H receptor density drives diversity in NK cell function during MCMV infection•NK cells undergo avidity selection during CMV infection•Ly49Hhi NK cells possess greater potential for cytotoxic and adaptive responses•Ly49Hlo NK cells possess greater potential for IFN-γ production NK cells are innate lymphocytes capable of “adaptive” responses after infection, but whether they undergo avidity selection is unknown. Adams et al. report that diversity in antigen receptor Ly49H expression drives NK cell functional heterogeneity during MCMV infection with high-avidity NK cells being selected to dominate the adaptive response.