Aims: The aim of this study was to evaluate the in-hospital and 1-year prognostic impact of diabetes and elevated blood glucose levels at hospital admission in patients with acute heart failure (HF). Methods and results: We studied a multinational cohort of 6926 hospitalized patients with acute HF enrolled in the European Society of Cardiology (ESC) and Heart Failure Association (HFA) Long-Term Registry, of whom 49.4% (n = 3422) had known or previously undiagnosed diabetes (defined as self-reported history, or medication use, or fasting glucose levels ≥7.0 mmol/L or haemoglobin A1c ≥6.5%). Compared with those without diabetes, patients with known or previously undiagnosed diabetes had higher cumulative rates of in-hospital mortality, 1-year mortality, and 1-year HF re-hospitalization that occurred independently of multiple clinical risk factors: in-hospital mortality [6.8 vs. 4.4%; adjusted hazard ratio (HR) 1.774; 95% confidence interval (CI) 1.282–2.456, P < 0.001], 1-year all-cause mortality (27.5 vs. 24%; adjusted HR 1.162; 95% CI 1.020–1.325, P = 0.024), and 1-year hospital re-admissions for HF (23.2 vs. 18.5%; adjusted HR 1.320; 95% CI 1.139–1.530, P < 0.001). Moreover, elevated admission blood glucose concentrations were powerfully prognostic for in-hospital mortality, but not for 1-year mortality or re-hospitalizations, in both patients with and without diabetes. Conclusions: Among patients hospitalized for acute HF, the presence of diabetes is independently associated with an increased risk of in-hospital mortality, 1-year all-cause mortality, and 1-year re-hospitalizations for HF, underscoring the need for more effective and personalized treatments of diabetes in this particularly high-risk patient population.