The rate of undiagnosed type 2 diabetes tends to increase in lean Koreans, while the triglyceride glucose (TyG) index has been proposed as a surrogate marker of peripheral insulin resistance. We investigated the longitudinal relationship between TyG and incident type 2 diabetes among apparently healthy Korean adults. We assessed 4285 lean adults without diabetes aged 40-69 years from the Korean Genome and Epidemiology Study. Participants were divided into 4 groups according to quartiles of TyG index, calculated as ln [fasting triglycerides (mg/dL) × fasting plasma glucose (mg/dL)/2]. We prospectively assessed the hazard ratios (HRs) with 95% confidence intervals (CIs) for incident type 2 diabetes, based on the American Diabetes Association criteria, using multivariate Cox proportional hazards regression models, over 12 years after the baseline survey. During the follow-up period, 631 (14.7%) participants had newly developed type 2 diabetes. The HRs of incident type 2 diabetes in each TyG index quartile were 1.00, 1.63 (95%CI, 1.18-2.24), 2.30 (95%CI, 1.68-3.14), and 3.67 (95%CI, 2.71-4.98), respectively, after adjusting for age, sex, body mass index, waist circumference, smoking status, alcohol intake, and physical activity. Higher TyG index precedes and significantly predicts type 2 diabetes among community-dwelling middle aged and elderly lean Koreans.