Background: The diagnostic cutoff points for indicators of type 2 diabetes mellitus (T2DM) in the pediatric population have not been defined thus far. Methods: A retrospective, single-center study was conducted from April 2003 to May 2016. We enrolled 236 overweight or obese children and adolescents aged 4–17 years. Thirty-nine (26.9%) of 145 patients had T2DM according to the oral glucose tolerance test results. Results: A glycated hemoglobin (HbA1c) level of 6.5% had a sensitivity and specificity of 87.2% and 98.5%, respectively, for detecting T2DM. The optimal HbA1c cutoff level for T2DM was >6.2% (94.7% sensitivity, 95.5% specificity). Conclusions: We observed that the use of an HbA1c level of 6.5% had a lower sensitivity for detecting T2DM than an HbA1c level of >6.2%.