Although the prognostic nutritional index (PNI) is reported as a prognosticator in patients with heart failure (HF), that is evaluated usually on one occasion, and any changes in PNI during hospitalization are not considered. This study aimed to assess between changes in the PNI during hospitalization and outcomes in patients with acute HF. We enrolled 141 patients (median age, 84 years, 75 male) admitted to our hospital for the treatment of acute HF. The PNI was calculated on admission and at discharge based on the original report. According to the PNI change during hospitalization, patients were classified as either improved (PNI at discharge ≥ PNI on admission) or deteriorated (PNI at discharge < PNI on admission). Primary outcomes were all-cause death or unplanned hospitalization due to HF within the first year. Forty-nine events occurred (19 deaths, 30 HF hospitalizations). The event-free survival rate determined by Kaplan–Meier analysis was significantly higher in patients in the improved group (log-rank test, P < 0.0001), regardless of the PNI value on admission. Multivariate analysis showed that younger age (HR 1.06, 95% CI 1.01–1.11, P = 0.016), higher body mass index (HR 0.90, 95% CI 0.82–0.98, P = 0.021) and the PNI in the improved group (HR 0.30, 95% CI 0.14–0.57, P = 0.0006) were independently associated with favorable outcomes. In conclusion, changes in nutritional status during hospitalization, evaluated using the PNI on admission and at discharge, were independently associated with 1-year outcomes in patients with acute HF.