We tested the hypothesis that supplementation of nicotinamide mononucleotide (NMN), a key NAD+ intermediate, increases arterial SIRT1 activity and reverses age-associated arterial dysfunction and oxidative stress. Old control mice (OC) had impaired carotid artery endothelium-dependent dilation (EDD) (60 ± 5% vs. 84 ± 2%), a measure of endothelial function, and nitric oxide (NO)-mediated EDD (37 ± 4% vs. 66 ± 6%), compared with young mice (YC). This age-associated impairment in EDD was restored in OC by the superoxide (O2-) scavenger TEMPOL (82 ± 7%). OC also had increased aortic pulse wave velocity (aPWV, 464 ± 31 cm s-1 vs. 337 ± 3 cm s-1 ) and elastic modulus (EM, 6407 ± 876 kPa vs. 3119 ± 471 kPa), measures of large elastic artery stiffness, compared with YC. OC had greater aortic O2- production (2.0 ± 0.1 vs. 1.0 ± 0.1 AU), nitrotyrosine abundance (a marker of oxidative stress), and collagen-I, and reduced elastin and vascular SIRT1 activity, measured by the acetylation status of the p65 subunit of NFκB, compared with YC. Supplementation with NMN in old mice restored EDD (86 ± 2%) and NO-mediated EDD (61 ± 5%), reduced aPWV (359 ± 14 cm s-1 ) and EM (3694 ± 315 kPa), normalized O2- production (0.9 ± 0.1 AU), decreased nitrotyrosine, reversed collagen-I, increased elastin, and restored vascular SIRT1 activity. Acute NMN incubation in isolated aortas increased NAD+ threefold and manganese superoxide dismutase (MnSOD) by 50%. NMN supplementation may represent a novel therapy to restore SIRT1 activity and reverse age-related arterial dysfunction by decreasing oxidative stress.