Heart failure with preserved ejection fraction (HFpEF) remains a major health concern with limited therapeutic options. Growing evidence supports the multiple benefits of ketones in heart disease, but their impact on HFpEF remains unknown. We investigated whether increasing ketones can help to manage HFpEF. Using the ZSF1 rat model of HFpEF, 16-week-old rats were randomly assigned to one of three subgroups: (i) control diet; (ii) ketogenic diet (KD); or (iii) control diet with added exogenous ketone salts (KS) in their drinking water for 10 weeks. We found that both KD and KS ameliorated the HFpEF phenotype by improving structural echocardiographic parameters, lowering glycaemia and lipid profiles, and reducing HFpEF-related fibrosis and hypertrophy without impacting in vivo diastolic function. Nevertheless, ex vivo cardiomyocyte preparations showed improved calcium handling and myofilament relaxation, suggesting benefits at the cellular level. Interestingly, KD still proved effective, despite the potentially adverse increase in fat mass. There was decreased myofilament Ca2+ sensitivity and normalized active and passive tension in both groups, especially KS. These results suggest that providing ketone through the diet or supplements could be a valuable strategy to complement HFpEF treatment. Given the well-known challenges of implementing dietary changes, exogenous KS offer a more practical and effective option to achieve these benefits. KEY POINTS: Ketogenic diet and ketone salts effectively reversed the cardiac structural impairments associated with the ZSF1 Obese heart failure with preserved ejection fraction (HFpEF) phenotype by ameliorating left ventricular mass. Both treatments reduced fibrosis and hypertrophy, leading to improved or, in the case of ketone salts, even reversed cardiomyocyte contractile and relaxation performance. Ketone salts also reversed HFpEF-related cardiomyocyte stiffness and prevented a reduction in the development of maximum force. Both treatments improved myofilament Ca2+ sensitivity. Both treatments also improved the metabolic profile, reducing hyperglycaemia, blood triglycerides and levels of NT-proBNP, a well-known biomarker of worsening heart failure.