Preclinical studies have revealed that the elevation of nicotinamide adenine dinucleotide (NAD+) levels on administration of an NAD+ precursor, nicotinamide mononucleotide (NMN), can mitigate aging-related disorders; however, human data are sparse. Therefore, we aimed to investigate whether the chronic oral supplementation of NMN can elevate blood NAD+ levels and alter physiological dysfunctions, including muscle weakness, in healthy elderly participants. We administered 250 mg NMN per day to aged men for 6 or 12 weeks (n=21 for 6 weeks, n=10 for 12 weeks) in a placebo-controlled, randomized, double blind, parallel-group trial. Chronic supplementation with NMN was well tolerated and did not cause any significant deleterious effect. Metabolomic analysis of whole blood demonstrated that the oral supplementation of NMN significantly increased the concentrations of NAD+ and NAD+ metabolites. Moreover, NMN significantly improved muscle strength and performance, which were evaluated using the 30-second chair stand test, walking speed, and grip strength, and it showed no significant effect on body composition. Thus, our evidence indicates that chronic oral NMN supplementation can be an efficient NAD+ booster for preventing aging-related muscle dysfunctions in humans.