Aims Limited therapeutic options are available for the management of atrial fibrillation/flutter (AF/AFL) with concomitant heart failure (HF) with preserved (HFpEF) and mildly reduced ejection fraction (HFmrEF). Dronedarone reduces the risk of cardiovascular events in patients with AF, but sparse data are available examining its role in patients with AF complicated by HFpEF and HFmrEF. Methods and results ATHENA was an international, multicentre trial that randomized 4628 patients with paroxysmal or persistent AF/AFL and cardiovascular risk factors to dronedarone 400 mg twice daily versus placebo. We evaluated patients with (i) symptomatic HFpEF and HFmrEF (defined as left ventricular ejection fraction LVEF >40%, evidence of structural heart disease, and New York Heart Association class II/III or diuretic use), (ii) HF with reduced ejection fraction (HFrEF) or left ventricular dysfunction (LVEF ≤40%), and (iii) those without HF. We assessed effects of dronedarone versus placebo on death or cardiovascular hospitalization (primary endpoint), other key efficacy endpoints, and safety. Overall, 534 (12%) had HFpEF or HFmrEF, 422 (9%) had HFrEF or left ventricular dysfunction, and 3672 (79%) did not have HF. Patients with HFpEF and HFmrEF had a mean age of 73 ± 9 years, 37% were women, and had a mean LVEF of 57 ± 9%. Over a mean follow‐up of 21 ± 5 months, dronedarone consistently reduced risk of death or cardiovascular hospitalization (hazard ratio 0.76; 95% confidence interval 0.69–0.84) without heterogeneity based on HF status (pinteraction >0.10). This risk reduction in the primary endpoint was consistent across the range of LVEF (as a continuous function) in HF without heterogeneity (pinteraction = 0.71). Rates of death, cardiovascular hospitalization, and HF hospitalization each directionally favoured dronedarone versus placebo in HFpEF and HFmrEF, but these treatment effects were not statistically significant in this subgroup. Conclusions Dronedarone is associated with reduced cardiovascular events in patients with paroxysmal or persistent AF/AFL and HF across the spectrum of LVEF, including among those with HFpEF and HFmrEF. These data support a rationale for a future dedicated and powered clinical trial to affirm the net clinical benefit of dronedarone in this population. Dronedarone for the treatment of atrial fibrillation with concomitant heart failure with preserved and mildly reduced ejection fraction: a post‐hoc analysis of the ATHENA trial.