Objective: A prolonged length of stay in the emergency department (EDLOS) is known to be associated with poorer outcomes in critically ill patients. However, this has not been proven in patients who visit the emergency department (ED) due to acute heart failure (AHF). We aimed to find out whether prolonged EDLOS is associated with major in-hospital complications in patients with AHF. Methods: This is a retrospective cohort study of AHF patients who were admitted to intensive care units (ICU) via the ED of a single academic hospital from January 2015 to December 2019. We divided the patients into two groups: EDLOS <24 hours and EDLOS≥24 hours. The primary outcome was major in-hospital complications, which included in-hospital death, application of continuous renal replacement therapy, or extracorporeal membrane oxygenation treatment. Secondary outcomes included in-hospital death, prolonged ICU stay (≥10 days), and prolonged hospital stay (≥14 days) excluding ED stay. Results: A total of 265 patients were enrolled. Of these 163 patients stayed in the ED for over 24 hours. The multivariable logistic analysis demonstrated that EDLOS ≥24 hours was independently associated with major in-hospital complications (odds ratio [OR], 3.296; 95% confidence interval [CI], 1.291-8.413; P=0.013). Analysis of the secondary outcomes showed that EDLOS ≥24 hours was associated with in-hospital death (OR, 2.607; 95% CI, 1.005-6.759; P=0.049) and prolonged hospital stay ≥14 days (OR, 2.458; 95% CI, 1.303-4.636; P=0.006). Conclusion Our study showed that in patients with AHF who visited ED and were admitted to the ICU, prolonged EDLOS was associated with major in-hospital complications.