We aimed to assess the association of overweight and obesity with diabetes, and the mediating role of hypertension in this association, among Chinese adults. This longitudinal study comprised adults who had participated in at least three survey waves of the China Health and Nutrition Survey. We extracted data on blood pressure, height, weight, and self-reported doctor-diagnosed diabetes. Overweight and obesity, hypertension, and diabetes emerged successively if the above-mentioned condition launched. Logistic regression models were used to assess the longitudinal associations of overweight and obesity and hypertension with diabetes. Mediation analyses were performed to test hypertension as a potential mediator of the above associations. In total, 6,996 adults [men: 46.2%; mean age (age range) at baseline: 39.5 (18–92) years] across China were included in this study. The combined prevalence of overweight and obesity at baseline was 41.5% (prevalence of overweight: 37.4%; prevalence of obesity: 4.1%). At a mean follow-up of 14.3 years, the incidence of hypertension and diabetes was 36.7% and 2.2%, respectively. The mean ages at onset of overweight and obesity, hypertension, and diabetes were 41.8, 53.4, and 56.1 years, respectively. Overweight and obesity and hypertension were independently associated with diabetes (all P < 0.001). Participants with overweight and obesity and hypertension were more likely to have subsequent diabetes than those without overweight and obesity and hypertension (odds ratio = 19.9; P < 0.001). Hypertension partially mediated the association between overweight and obesity and diabetes (percentage of mediation = 10.4%; P < 0.001). Our findings highlight the need for early identification of and immediate intervention for the disease chain of overweight and obesity, hypertension, and diabetes.