Interest in pharmacological agents capable of increasing cellular NAD+ concentrations has stimulated investigations of nicotinamide riboside (NR) and nicotinamide mononucleotide (NMN). NR and NMN require large dosages for effect. Herein, we describe synthesis of dihydronicotinamide riboside (NRH) and the discovery that NRH is a potent NAD+ concentration-enhancing agent, which acts within as little as 1 h after administration to mammalian cells to increase NAD+ concentrations by 2.5-10-fold over control values. Comparisons with NR and NMN show that in every instance, NRH provides greater NAD+ increases at equivalent concentrations. NRH also provides substantial NAD+ increases in tissues when administered by intraperitoneal injection to C57BL/6J mice. NRH substantially increases NAD+/NADH ratio in cultured cells and in liver and no induction of apoptotic markers or significant increases in lactate levels in cells. Cells treated with NRH are resistant to cell death caused by NAD+-depleting genotoxins such as hydrogen peroxide and methylmethane sulfonate. Studies to identify its biochemical mechanism of action showed that it does not inhibit NAD+ consumption, suggesting that it acts as a biochemical precursor to NAD+ Cell lysates possess an ATP-dependent kinase activity that efficiently converts NRH to the compound NMNH, but independent of Nrk1 or Nrk2. These studies identify a putative new metabolic pathway to NAD+ and a potent pharmacologic agent for NAD+ concentration enhancement in cells and tissues.