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Background Many hospitalized patients with heart failure and reduced ejection fraction (HFrEF) have a slow heart rate at discharge, and the effect of β‐blockers may be reduced in those patients. We sought to examine the variable effect of β‐blockers on clinical outcomes according to the discharge heart rate of hospitalized HFrEF patients. Methods and Results The KorAHF (Korean Acute Heart Failure) registry consecutively enrolled 5625 patients hospitalized for acute heart failure. In this analysis, we included patients with HFrEF (left ventricular ejection fraction ≤40%). Slow heart rate was defined as <70 beats per minute regardless of the use of β‐blockers. The primary outcome was 1‐year all‐cause postdischarge death according to heart rate. Among 2932 patients with HFrEF, 840 (29%) had a slow heart rate and 56% received β‐blockers at discharge. Patients with slow heart rates were older and had lower 1‐year mortality than those with high heart rates (P<0.001). A significant interaction between discharge heart rate and β‐blocker use was observed (P<0.001 for interaction). When stratified, only patients without a β‐blocker prescription and with a high heart rate showed higher 1‐year mortality. In a Cox‐proportional hazards regression analysis, β‐blocker prescription at discharge was associated with 24% reduced risk for 1‐year mortality in patients with high heart rates (hazard ratio: 0.76; 95% CI, 0.61–0.95) but not in those with slow heart rates (hazard ratio: 1.02; 95% CI, 0.68–1.55). Conclusions Many patients with acute heart failure have slow discharge heart rates, and β‐blockers may have a limited effect on HFrEF and slow discharge heart rate. Clinical Trial Registration URL: http://www.clinicaltrial.gov. Unique identifier: NCT01389843.