Abstract Background More than half of the patients admitted for acute decompensated heart failure (ADHF) in Japan are over 80 years of age. Both Japanese and European heart failure guidelines recommend the use of renin-angiotensin system inhibitors (RASIs) in patients with heart failure with reduced ejection fraction (HFrEF). However, there is a paucity of data on the effect of RASIs in elderly patients with HFrEF, because elderly patients have been excluded from large clinical trials. Purpose The purpose of the current study was to clarify the effect of RASIs in elderly patients with HFrEF. Methods This was a post-hoc analysis of a prospective, observational, multicentre cohort study that enrolled consecutive patients admitted with ADHF to 19 secondary and tertiary hospitals in Japan between October 2014 and March 2016. Patients who were ≥ 80 years old, with left ventricular ejection fraction (LVEF) < 40% and discharged alive were included in the present study. RASIs were defined as either ACE inhibitors or angiotensin II receptor blockers. The primary endpoint was a composite of all-cause death and heart failure (HF) hospitalisation. Results Among 518 patients enrolled, 288 patients received RASIs at discharge and 230 patients did not. The median follow-up period was 335 days (interquartile range; 124 – 499 days). Patients receiving RASIs were younger (85.7 ± 4.4 vs. 86.5 ± 4.4 years of age, p=0.03), had a higher body mass index (BMI; 21.4 ± 3.3 vs. 20.6 ± 3.7 kg/m², p=0.01) and better renal function (eGFR; 45.4 ± 21.5 vs. 38.2 ± 19.2 ml/min/1.73m², p<0.001). Patients receiving RASIs had a higher LVEF (30.9 ± 6.3 vs. 28.5 ± 7.1%, p<0.001). The use of beta-blockers at discharge was more prevalent in patients receiving RASIs (77.8% vs. 60.4%, p<0.001), but the use of mineralocorticoid receptor antagonists was not (50.0% vs. 42.2%, p=0.08). The cumulative incidence of all-cause death or HF hospitalisation one year after discharge was significantly lower in patients receiving RASIs (40.1% vs. 59.5%, p<0.001). Even after adjusting for confounders, the risk of a composite of all-cause death and HF hospitalisation was significantly lower in patients receiving RASIs (adjusted hazard ratio 0.59, 95% confidence interval 0.45-0.77, p<0.001). There was no interaction between the effect of RASIs and subgroup factors such as age, BMI, dementia, malignancy, frailty, and renal function. Conclusion Use of RASIs at discharge was associated with lower incidence of all-cause death or HF hospitalisation in elderly patients with HFrEF.