It is not clear whether tolvaptan is safe and effective irrespective of various underlying clinical conditions including the functional ventricle morphology, chromosomal abnormalities, and renal function after complex pediatric congenital heart disease surgery. Also, the appropriate dose of tolvaptan in these patients has not been previously identified. We retrospectively assessed the urine volume, body weight, patient clinical characteristics, laboratory data, and vital signs before and on days 1 and 7 of the tolvaptan administration after congenital heart disease surgery. Also, we assessed the relationship between the tolvaptan dose and its effects. A total of 86 patients were included the study. The mean time from the surgery to the tolvaptan administration was 23.5 ± 3.7 days. After administering tolvaptan, the urine volume significantly increased and body weight significantly decreased from baseline by days 1 and 7 (p < 0.0001). The urine volume significantly increased more in the survivors than the deceased. Of the 22 patients who had low serum sodium concentrations at baseline, 20 had an increased serum sodium concentration on day 7. The clinical effect of tolvaptan was not affected by the functional ventricle morphology, chromosomal abnormalities, or renal function. There was a positive correlation between the tolvaptan dose and change in the urine volume until a tolvaptan dose of up to 0.3 mg/kg/day but not at more than 0.3 mg/kg/day. Tolvaptan administration is safe and effective after congenital heart disease surgery irrespective of various underlying clinical conditions. Though the urine volume tends to increase until a tolvaptan dose of up to 0.3 mg/kg/day in pediatric congenital heart disease patients, there was no further benefit with more than 0.3 mg/kg/day.