Heart failure (HF) with left ventricular diastolic dysfunction is a growing global concern. This study evaluated myocardial oxidized nicotinamide adenine dinucleotide (NAD⁺) levels in human systolic and diastolic HF and in a murine model of HF with preserved ejection fraction, exploring NAD⁺ repletion as therapy. We quantified myocardial NAD⁺ and nicotinamide phosphoribosyltransferase levels, assessing restoration with nicotinamide riboside (NR). Findings show significant NAD⁺ and nicotinamide phosphoribosyltransferase depletion in human diastolic HF myocardium, but NR successfully restored NAD⁺ levels. In murine HF with preserved ejection fraction, NR as preventive and therapeutic intervention improved metabolic and antioxidant profiles. This study underscores NAD⁺ repletion's potential in diastolic HF management.