Background: Telomerase reverse transcriptase (TERT) plays an important role in cancer cells. Recently, mutations in the promoter of the TERT gene were identified in several types of cancers, and have also been reported as indicators of poor outcome in some of these patients. The majority of reported mutations were located at two hotspots, -124C>T and -146C>T. The aim of this study was to examine the incidence of TERT promoter mutations in ovarian clear cell carcinomas (OCCCs) in the Japanese population. Materials and Methods: The authors screened for TERT promoter mutations (focusing on the two hotspots) in 40 patients with OCCCs: 11 patients with endometrioid carcinoma, and 10 patients with high-grade serous carcinoma. They further measured the effect of TERT promoter mutations in an OCCC cell line (ES-2) using the luciferase reporter assay. Results: The incidence of hotspot mutations was 2.5% (1/40) in the patients with OCCCs, but was 0% (0/11 and 0/10) in patients with endometrioid carcinoma and high-grade serous carcinoma. The luciferase assay confirmed that the identified hotspot mutation is a gain-of-function mutations in OCCC cells. Conclusion: TERT promoter mutations increased the activity of OCCC cells. However, the frequency of these mutations in OCCCs appears to be very rare in the Japanese population. The present data indicate that TERT promoter mutations do not play an important role in OCCCs in the Japanese population.