Patients admitted for acute heart failure (AHF) experience high rates of in‐hospital and post‐discharge morbidity and mortality despite current therapies. Serelaxin is recombinant human relaxin‐2, a hormone with vasodilatory and end‐organ protective effects believed to play a central role in the cardiovascular and renal adaptations of human pregnancy. In the phase 3 RELAX‐AHF trial, serelaxin met its primary endpoint of improving dyspnoea through day 5 in patients admitted for AHF. Compared to placebo, serelaxin also reduced worsening heart failure (WHF) by 47% through day 5 and both all‐cause and cardiovascular mortality by 37% through day 180. RELAX‐AHF‐2 ( ClinicalTrials.gov NCT01870778) is designed to confirm serelaxin's effect on these clinical outcomes. RELAX‐AHF‐2 is a multicentre, randomized, double‐blind, placebo‐controlled, event‐driven, phase 3 trial enrolling ∼6800 patients hospitalized for AHF with dyspnoea, congestion on chest radiograph, increased natriuretic peptide levels, mild‐to‐moderate renal insufficiency, and systolic blood pressure ≥125 mmHg. Patients are randomized within 16 h of presentation to 48 h intravenous infusions of serelaxin (30 µg/kg/day) or placebo, both in addition to standard of care treatments. The primary objectives are to demonstrate that serelaxin is superior to placebo in reducing: (i) 180 day cardiovascular death, and (ii) occurrence of WHF through day 5. Key secondary endpoints include 180 day all‐cause mortality, composite of 180 day combined cardiovascular mortality or heart failure/renal failure rehospitalization, and in‐hospital length of stay during index AHF. The results from RELAX‐AHF‐2 will provide data on the potential beneficial effect of serelaxin on cardiovascular mortality and WHF in selected patients with AHF.