We propose a new anthropometric index, weight-adjusted-waist index (WWI), to assess adiposity by standardizing waist circumference (WC) for weight. WWI, calculated as WC (cm) divided by the square root of weight (kg) (cm/√kg), was measured from 465,629 subjects in the Korean nationwide cohort (2008-2013). Cox regression analysis was used to compare WWI with BMI, WC, waist-to-height ratio (WHtR), and a body shape index (ABSI) for cardiometabolic morbidity and mortality risk in diagnostic and prognostic prediction models. For incident hypertension, type 2 diabetes and cardiovascular disease (CVD), BMI had the strongest predictive power, followed by WWI and WC. However, WWI showed the best predictive performance for CVD mortality. Also, a linear positive association between adiposity indices and cardiovascular and all-cause mortality was only shown in WWI and ABSI, not BMI, WC and WHtR which showed inverse J-shaped patterns. In the test of joint effects of each index, WWI combined with BMI was the strongest in both diagnostic and prognostic models. WWI is a unique adiposity index that shows linear positive association with both cardiometabolic morbidity and mortality. It also predicts incident cardiometabolic disease, cardiovascular and all-cause mortality risk with excellence in predictive power, especially when combined with BMI.