BACKGROUND: Red blood cell transfusion is frequently prescribed in the emergency department for patients with gastrointestinal bleeding (GIB), but the association of time to transfusion with patient outcome has not been thoroughly evaluated. METHODS: A retrospective cohort study analyzed adult patient data with GIB who visited the emergency department of single university-affiliated hospital between January 2016 and April 2022. The associations of time to transfusion and patient outcomes, 30-day and in-hospital mortality, were assessed. RESULTS: Among a total of 2,284 patients, 1,395 (61.1%) received red blood cell transfusion within 4 h of emergency department admission. Analysis of the time to transfusion showed the association between late transfusion (transfusion after 4 h) and the risk of 30-day mortality (adjusted hazard ratio, 1.65, 95% CI 1.17-2.32, CONCLUSIONS: In this study, a longer time to red blood cell transfusion was associated with an increased risk of 30-day and in-hospital mortality of patients with GIB, especially in those with upper GIB, nonvariceal bleeding and a low haemoglobin level. In the emergency department, prompt red blood cell transfusion decisions for patients with GIB may improve patient outcomes.