We investigated the incidence of diagnosed diabetes in South Korean adults (aged ≥20 years) by analyzing data for the National Health Insurance Service–National Sample Cohort. From 2006 to 2015, the overall incidence rate of diagnosed diabetes decreased by approximately 0.1% per year until 2015. Although, this trend was observed in individuals aged 40 years or over, the rate increased slightly in the 20–29 and 30–39 years age groups, from 0.5 to 0.7 and 2.0 to 2.6 per 1,000 individuals, respectively. The proportion of obese young adults with diabetes increased remarkably, from 51.4% in 2006 to 72.4% in 2015. Thus, young adults need early identification and weight-control strategies to prevent diabetes.