With the aging society, the number of very-elderly (VE) patients with acute decompensated heart failure (ADHF) is increasing. Although tolvaptan is recommended for patients with ADHF in whom conventional diuretic therapy is ineffective, few reports exist on VE patients over 90years of age. Therefore, we aimed to evaluate the clinical effectiveness and adverse events associated with tolvaptan in VE patients with ADHF. From January 2011 to December 2018, we retrospectively studied 180 patients with ADHF who were first administered tolvaptan during hospitalization. Patients were divided into two groups, namely, VE patients who were ≥ 90years of age (n = 32) and not-VE patients (NVE) who were < 90years of age (n = 148). The primary effective endpoints were the total urine volume and change in body weight. The safety endpoints evaluated were the incidence of hypernatremia (≥ 150mEq/L) and worsening renal function (WRF) at any time during hospitalization. The median [interquartile range] patient age was 93 [91-94] years in the VE group and 80 [69-85] years in the NVE group. The mean dose of tolvaptan for the first week of administration was similar between groups (7.9 ± 5.0mg, VE group; 7.3 ± 3.7mg, NVE group; p = 0.52). There were no significant differences between the two groups in the total urine volume at 24h (1901 ± 666mL, VE group; 2101 ± 1167mL, NVE group; p = 0.33) and that at 48h (3707 ± 1274mL, VE group; 4195 ± 1990mL, NVE group; p = 0.19) and in the mean change in body weight (-2.5 ± 2.0kg, VE group; -2.7 ± 2.4kg, NVE group; p = 0.70). The median duration of hospitalization was 24 [20-9] and 31 [20-42] days in the VE and NVE groups, respectively (p = 0.67). The incidence of hypernatremia (6.3% (2/32), VE group; 3.4% (5/148), NVE group; p = 0.61) and WRF (25.0% (8/32) VE group; 19.6% (29/148), NVE group; p = 0.31) was similar between the groups. In conclusion, tolvaptan has similar clinical effectiveness in increasing urine volume and decreasing body weight, without increased adverse events, in VE patients with ADHF who were ≥ 90years of age compared to NVE patients with ADHF.