Heterozygous RFX6 mutation has emerged as a potential cause of maturity‐onset diabetes mellitus of the young (MODY). A 16‐year‐old female was diagnosed with diabetes by her family doctor and was referred to our institution for genetic examination. Genetic testing revealed a novel RFX6 heterozygous mutation (NM_173560: exon17: c.1954C>T: p.R652X) in the patient and in her mother and brother. She had no islet‐specific autoantibodies and showed a reduced meal‐induced response of insulin, glucose‐dependent insulinotropic polypeptide, and glucagon‐like peptide‐1, which is consistent with the phenotype of MODY due to heterozygous RFX6 mutation. In conclusion, we report a case of MODY due to a novel heterozygous mutation, p.R652X.