The pathogenesis of heart failure with a preserved ejection fraction (HFpEF) is unclear. Myocardial fibrosis, inflammation, and cardiac hypertrophy have been suggested to contribute to the pathogenesis of HFpEF. Cardiosphere-derived cells (CDCs) are heart-derived cell products with antifibrotic and anti-inflammatory properties. This study tested whether rat CDCs were sufficient to decrease manifestations of HFpEF in hypertensive rats. Starting at 7 weeks of age, Dahl salt-sensitive rats were fed a high-salt diet for 6 to 7 weeks and randomized to receive intracoronary CDCs or placebo. Dahl rats fed normal chow served as controls. High-salt rats developed hypertension, left ventricular (LV) hypertrophy, and diastolic dysfunction, without impairment of ejection fraction. Four weeks after treatment, diastolic dysfunction resolved in CDC-treated rats but not in placebo. The improved LV relaxation was associated with lower LV end-diastolic pressure, decreased lung congestion, and enhanced survival in CDC-treated rats. Histology and echocardiography revealed no decrease in cardiac hypertrophy after CDC treatment, consistent with the finding of sustained, equally-elevated blood pressure in CDC- and placebo-treated rats. Nevertheless, CDC treatment decreased LV fibrosis and inflammatory infiltrates. Serum inflammatory cytokines were likewise decreased after CDC treatment. Whole-transcriptome analysis revealed that CDCs reversed changes in numerous transcripts associated with HFpEF, including many involved in inflammation and/or fibrosis. These studies suggest that CDCs normalized LV relaxation and LV diastolic pressure while improving survival in a rat model of HFpEF. The benefits of CDCs occurred despite persistent hypertension and cardiac hypertrophy. By selectively reversing inflammation and fibrosis, CDCs may be beneficial in the treatment of HFpEF.