Conventional diuretic therapy for low-flow (LF) severe aortic stenosis (SAS) often has an inadequate effect or causes hemodynamic instability. Tolvaptan is used for acute heart failure in addition to conventional diuretics, and it does not cause intravascular dehydration. This study aimed to retrospectively investigate the safety and efficacy of tolvaptan in the acute phase in 56 consecutive patients with SAS and compared LF-SAS with normal-flow (NF) SAS. The primary endpoints were adverse clinical events (death, worsening heart failure, worsening renal failure, fatal arrhythmia, cardiogenic or hypovolemic shock, and use of inotropic agents) and the volume of urine within 48 h of tolvaptan administration. Among 56 patients, 16 had LF-SAS (29%), and 40 had NF-SAS (71%). Severe adverse clinical events were not observed 48 h after tolvaptan administration. In both groups, the urine volume significantly increased after tolvaptan administration in comparison to 24 h before tolvaptan administration (both, p < 0.01). There were no changes in the urine volume during the initial 24 and 48 h. In the LF-SAS group, tolvaptan resulted in a significant decrease in fluid balance during the initial 24 and 48 h compared to 24 h before tolvaptan administration (p < 0.05). Adding tolvaptan to conventional treatment is safe and effective without renal dysfunction and hypotension in patients with SAS, including those with LF.