Left ventricular (LV) diastolic dysfunction is a hallmark of Heart Failure with preserved Ejection Fraction (HFpEF), an escalating global health challenge. We demonstrated selective depletion of the oxidized form of nicotinamide adenine dinucleotide (NAD+) and the rate-limiting enzyme of the NAD+ biosynthetic salvage pathway, nicotinamide phosphoribosyltransferase (NAMPT), in human myocardium with LV diastolic dysfunction. We showed that NAD+ can be replenished in human myocardium with diastolic impairment ex vivo, despite reduced NAMPT expression. In a murine model of HFpEF [a combination exposure to high-fat diet (HFD) and L-NG-Nitro arginine methyl ester (L-NAME)], we compared the benefits of NAD+ precursor supplementation versus dietary intervention. We tested NAD+ repletion by nicotinamide riboside (NR) supplementation using two clinically-relevant strategies: 1) Prophylactic NR repletion before HFpEF onset, and 2) Therapeutic NR repletion after the development of HFpEF. We found that dietary intervention (replacement of HFD and L-NAME with healthy diet) restored myocardial insulin-dependent glucose uptake and glycolysis but did not rescue HFpEF. In contrast, both NAD+ repletion strategies prevented or rescued HFpEF, respectively, plausibly due to restoration of myocardial iron homeostasis, recoupling of glycolysis to the TCA cycle, and upregulation of antioxidant defense.