Tolvaptan, a vasopressin receptor antagonist, has been shown to be effective in the treatment of renal cysts in ADPKD. However, tolvaptan is not indicated for pediatric patients, and reports of its use are rare, making its efficacy and adverse reactions unclear. Herein, we present the case of an 11-year-old girl who had vitiligo from birth. She was diagnosed with West syndrome at 6months of age and tuberous sclerosis at 2years of age. At the age of 6years, an abdominal magnetic resonance imaging (MRI) revealed multiple bilateral renal cysts, and she was diagnosed with ADPKD. Abdominal MRI scans performed at 10years and 11years showed rapid renal cyst enlargement, and the renal prognosis was judged to be poor. The patient was treated with tolvaptan to delay cyst exacerbation. There were no apparent adverse events after the initiation of treatment, and the MRI performed 12months after treatment initiation showed that renal cyst enlargement was suppressed. The results suggest that tolvaptan may be effective in pediatric patients with severe ADPKD who have rapidly enlarging renal cysts, although evaluation of renal cyst enlargement and side effects should be continued.