Since acute kidney injury (AKI) is not always related to mortality in patients with acute decompensated heart failure (ADHF), the aim of this study was to focus on onset time of AKI and its clinical importance. A total of 371 ADHF patients were included. The impact of AKI (≥0.3 mg/dl or 1.5-fold increase in serum creatinine level within 48 h) with early onset (≤4 days from admission) or late onset (≥5 days from admission) was assessed. AKI occurred in 99 patients, who were divided into two groups according to the median onset time of AKI: 50 with early onset of AKI and 49 with late onset of AKI. The maximum increase in serum creatinine level from admission was greater in patients with late onset of AKI than in patients with early onset of AKI (p = 0.012). Patients with late onset of AKI had a higher 12-month mortality rate than that in patients with early onset of AKI (log-rank test, p = 0.014). Late onset of AKI was an independent predictor of mortality (hazard ratio: 3.39, 95 % confidence interval: 1.84–6.18, p < 0.001). Late onset of AKI was associated with high blood urea nitrogen level at admission and intravenous administration of dobutamine. In conclusion, late onset of AKI related to high blood urea nitrogen level and intravenous administration of dobutamine, but not early onset of AKI, is linked to high mortality rate. Onset time of AKI may be useful for risk stratification of mortality in ADHF patients developing AKI.