Maturity-onset diabetes of the young (MODY) is a rare, autosomal dominant disease characterized by non-ketogenic diabetes mellitus (DM). MODY type 4, caused by <i>PDX1</i> mutation, is a very rare subtype of MODY, especially in Korea. We report a case of a 10-year-old, nonobese girl with a family history of type 2 DM. After diagnosis, the patient’s serum glucose level was well controlled using metformin monotherapy; however, the glycated hemoglobin level increased to 9.0% approximately 2 years after treatment. No obesity or lifestyle problems were observed, and serum fasting C-peptide level was within the normal range. Furthermore, no islet-related autoantibodies were detected. A genetic screening for MODY using a next-generation sequencing panel was performed, and a likely heterozygous pathogenic <i>PDX1</i> mutation (p.Gly246ArgfsTer21) was identified. The <i>PDX1</i> variant was not detected in her mother, implying that the mutation had arisen <i>de novo</i> in the proband. She was prescribed insulin degludec in addition to metformin therapy, which improved her hyperglycemia. This report presents a novel MODY type 4 phenotype and highlights the importance of genetic screening in patients with MODY characteristics.