Klinefelter syndrome (KS) is a chromosomal abnormality that is described to be one of the most common reasons for male infertility. The clinical symptoms of this disease are not typical in childhood, and it can also be accompanied by metabolic syndrome in adolescence and adulthood in addition to sexual developmental abnormalities. A case of KS with elevated blood glucose as the chief complaint was reported in this article. He was diagnosed as having diabetes mellitus in the beginning and then diagnosed as having KS complicated with type 2 diabetes mellitus after medical history inquiry and further examinations. The case suggests that clinicians need to improve their understanding of KS in order to provide diagnosis and treatment in time.