Sudden death is the most common mode of exodus in patients with heart failure and preserved ejection fraction (HFpEF). Cardiosphere-derived cells (CDCs) reduce inflammation and fibrosis in a rat model of HFpEF, improving diastolic function and prolonging survival. We tested the hypothesis that CDCs decrease ventricular arrhythmias (VAs) and thereby possibly contribute to prolonged survival. Dahl salt-sensitive rats were fed a high-salt diet to induce HFpEF. Allogeneic rat CDCs (or phosphate-buffered saline as placebo) were injected in rats with echo-verified HFpEF. CDC-injected HFpEF rats were less prone to VA induction by programmed electrical stimulation. Action potential duration (APD) was shortened, and APD homogeneity was increased by CDC injection. Transient outward potassium current density was upregulated in cardiomyocytes from CDC rats relative to placebo, as were the underlying transcript (Kcnd3) and protein (Kv4.3) levels. Fibrosis was attenuated in CDC-treated hearts, and survival was increased. Sudden death risk also trended down, albeit nonsignificantly. CDC therapy decreased VA in HFpEF rats by shortening APD, improving APD homogeneity, and decreasing fibrosis. Unlike other stem/progenitor cells, which often exacerbate arrhythmias, CDCs reverse electrical remodeling and suppress arrhythmogenesis in HFpEF.