NAD + precursors prevent COVID-19 plasma-induced endothelial cell dysfunction and hold promise as therapies for COVID-19-related vascular dysfunction...Co-incubation with NR abolished COVID-19-induced reductions in NO production and oxidative stress (both p > 0.05 vs. control). Co-treatment with NMN produced similar results. Our findings suggest the circulating milieu of patients with COVID-19 promotes endothelial cell dysfunction, characterized by lower NO bioavailability, greater ROS bioactivity, and NAD(+) depletion. Supplementation with NAD(+) precursors may exert a protective effect against COVID-19-evoked endothelial cell dysfunction and oxidative stress.