The effect of early use of tolvaptan (TLV) for acute decompensated heart failure (ADHF) is unclear. We investigated the relationship between early use of TLV and the length of hospital stay. 369 consecutive ADHF patients who received TLV during hospitalization between February 2011 and June 2016 were initially enrolled. Patients who died in hospital, transferred hospital or clinical scenario 4 or 5 were excluded. We analyzed 247 ADHF patients. We evaluated the relationship between the length of hospital stay and the following findings: blood pressures, heart rate, New York Heart Association classification, and blood tests on admission. Moreover, we also evaluated treated agents and TLV initiated days from admission. TLV initiated days was statistically associated with the length of hospital stay (r = 0.625, P < 0.001). We compared the early use (within 4 days) vs delayed use of TLV (5 days or later), because the median of time until commencement of TLV from hospitalization was 4 days. The length of hospital stay in the delayed use group was significantly longer than early use group (31.9 ± 20.4 and 21.0 ± 12.9 days, P < 0.001). However, there was no difference in the length of hospital stay after initiation of TLV in both groups. Moreover, we investigated the factors related to the long-term hospitalization (hospital stay of median length or more). Multivariate analysis showed that TLV initiated days was independently related to the long-term hospitalization (odds ratio 1.32, 95% confidence interval 1.13–1.53, P < 0.001). Early use of TLV was related to the length of hospital stay for ADHF patients.