To identify those who might benefit from weight reduction within a large population of obese individuals, Japan Society for the Study of Obesity (JASSO) advocated the concept of “obesity disease.” Here we summarize the definition, criteria, and core concepts for the management of obesity disease based on JASSO’s latest guideline. JASSO defines obesity as excessive fat storage in adipose tissue associated with a BMI of ≥25 kg/m2. The threshold BMI of obesity is low as compared to Western countries given that Japanese individuals tend to develop obesity-related health disorders at lower BMI. Obesity with a BMI of ≥35 kg/m2 is referred to as “high-degree obesity” as treatment strategies vary based on the degree of obesity. Obesity is diagnosed as “obesity disease” if accompanied by any of the 11 specific obesity-related health disorders that weight reduction can prevent or alleviate, or if it meets the criteria for visceral fat obesity with a visceral fat area of ≥100 cm2. The initial weight reduction goals for high-degree obesity disease range from 5% to 10% of their current body weight, depending on the associated health disorders. That for those with obesity disease who do not qualify as high-degree is 3% or more. If these initial goals are not achieved, intensifying dietary therapy or introducing drug therapy (or both) may be necessary. While surgical treatment is primarily indicated for high-degree obesity disease, it might be appropriate for cases of obesity disease with a BMI <35 kg/m2, depending on the accompanying health disorders. Enhancing the quality of life for individuals with obesity or obesity disease necessitates a broader societal approach, emphasizing the resolution of related stigma.