Background: Type 2 diabetes mellitus (T2DM) has traditionally been considered to affect mainly the elderly; however, the age at diagnosis has gradually reduced in recent years. Although the incidence of young-onset T2DM is increasing, it is still not fully clear the onset characteristics and risk factors of early-onset T2DM. The aim of this study was to describe the initiating characteristics of early-onset T2DM in Chinese patients and evaluate the risk factors for diabetes mellitus. Methods: This cross-sectional controlled study was performed using a questionnaire survey method in outpatients of multiple centers in China. A total of 1545 patients with T2DM with an age at onset of <40 years were included, and the control group consisted of subjects aged <40 years with normal blood glucose level. Results: In patients with young-onset T2DM, the mean age and initial hemoglobin 1Ac at diagnosis were 32.96 +/- 5.40 years and 9.59 +/- 2.71%, respectively. Most of the patients were obese, followed irregular diet pattern and sedentary lifestyle, had life or work pressure, and had a family history of diabetes mellitus. Compared with subjects with normal blood glucose level, logistic regression analysis showed that waist-to-hip ratio (odds ratio [OR] 446.99, 95% confidence interval [CI] 42.37-4714.87), family history of diabetes mellitus (OR 23.46, CI 14.47-38.03), dyslipidemia (OR 2.65, CI 1.54-4.56), diastolic blood pressure (OR 1.02, CI 1.00-1.04), and body mass index (OR 0.95, CI 0.92-0.99) are independent factors for early-onset T2DM. Conclusions: We observed that abdominal obesity, family history of diabetes mellitus, and medical history of hypertension and dyslipidemia are independent risk factors for early-onset T2DM. It is, therefore, necessary to apply early lifestyle intervention in young people with risk of diabetes mellitus.