The prevalence of obesity has consistently increased worldwide, and many obesity-related diseases are emerging as major health problems. Body mass index (BMI) is used to define obesity and is highly correlated with body fat mass. Moreover, obesity-related morbidities increase linearly with the increase in BMI. The Korean Society for the Study of Obesity defined overweight as a BMI ≥23 kg/m2 and obesity as a BMI ≥25 kg/m2, based on a significant increase in obesity-related diseases. A waist circumference of ≥90 cm in men and ≥85 cm in women are defined as abdominal obesity, which is also correlated with obesity-related diseases. These diagnostic criteria are the same as in the previous version; however, the updated guidelines put greater emphasis on the use of morbidity as the basis for obesity and abdominal obesity diagnoses. These new guidelines will help to identify and manage high-risk groups for obesity-related comorbidities among Korean adults.