Objective Sodium-glucose co-transporter-2 inhibitors (SGLT2is), such as dapagliflozin, have a diuretic effect, and their early initiation to treat acute heart failure (AHF) may improve outcomes; however, the significance of the timing of starting dapagliflozin after hospital admission remains unclear. Methods We performed a post hoc analysis of a prospective, observational registry. Participants were divided into the early (E) group and late (L) group using the median time to the initiation of dapagliflozin (6 days) as the cut-off. We evaluated the relationship between the time to the initiation of dapagliflozin after hospital admission and patient characteristics and the length of the hospital stay. Patients Study subjects were 118 patients with AHF admitted between January 2021 and April 2022 who were started on dapagliflozin treatment (10 mg/day). Results Patients were divided into the E group (n=63) and L group (n=55). The HF severity as evaluated by the New York Heart Association class and the N-terminal pro-brain natriuretic peptide level was not significantly different between the groups. The time to the initiation of dapagliflozin and length of hospital stay showed a significant positive correlation (p<0.001, r=0.46). The hospital stay was significantly shorter in group E median, 16.5 days; interquartile range (IQR): 13-22 days than in group L (median, 22 days; IQR: 17-27 days; p=0.002). A multivariate logistic regression analysis showed that the early initiation of dapagliflozin was independently associated with a shorter hospital stay, even after multiple adjustments. Conclusion Early initiation of dapagliflozin after hospital admission is associated with a shorter hospital stay, suggesting it is a key factor for shortening hospital stays.