The World Obesity Federation is urging governments to act because it predicts just over half of the global population will be classed as obese or overweight by 2035.
Its new report says more than four billion people will be affected, and rates are rising particularly quickly among children.
It says almost all of the countries expected to see the greatest increases are low or middle-income countries in Africa and Asia.
Reasons include trends in dietary preferences towards more highly processed foods, greater levels of sedentary behaviour, weaker policies to control food supply and marketing, and less well-resourced healthcare services to assist in weight management and health education.
The federation says the financial cost of obesity will amount to $4tn annually by 2035.