Nicotinamide mononucleotide (NMN), a key nicotinamide adenine dinucleotide (NAD+) intermediate, has been shown to ameliorate various pathologies in elderly mouse disease models. Natural killer (NK) cells are important innate immune cells; however, their functions decline with aging. In this study, we examined the effect of NMN treatment on NK cells in mice. Intraperitoneal administration of NMN augmented NK cell cytotoxic activity in both young and elderly B6 mice as well as young BALB/c mice. Oral administration of NMN also increased NK cell cytotoxicity in elderly B6 and BALB/c mice. However, the NK cell population was not increased in the mice whose NK cell cytotoxic activity was activated by NMN. Interestingly, NMN administration did not augment NK cell cytotoxic activity in IFN-γ deficient mice. These results suggest that NMN administration augments NK cell cytotoxic activity, but not cell number, in a manner dependent on IFN-γ in both young and elderly mice.