This equates to one-third of the world's population carrying excess weight, fueled by urbanization, poor diets and reduced physical activity.
The United States has the greatest percentage of obese children and young adults, at 13%, while Egypt led in terms of adult obesity, with almost 35%, among the 195 countries and territories included in the study.
While 2.2 billion people were obese or overweight in 2015, more than 710 million of them were classed as obese, with 5% of all children and 12% of adults fitting into this category.
An increasing number globally are dying from health problems linked to being overweight, such as cardiovascular disease, said the study,
published Monday in the New England Journal of Medicine.
Almost 40% of the 4 million dying as a result of their higher body mass index were not yet obese, highlighting that deaths are occurring almost as often in those considered overweight as those considered obese.
Body mass index is the ratio between a person's weight and height; a BMI of 25 to 29.9 is considered overweight, while anything over 30 is obese.
"People who shrug off weight gain do so at their own risk -- risk of cardiovascular disease, diabetes, cancer, and other life-threatening conditions," said Dr. Christopher Murray, director of the Institute for Health Metrics and Evaluation at the University of Washington, who worked on the study.
"Those half-serious New Year's resolutions to lose weight should become year-round commitments to lose weight and prevent future weight gain," he said in a statement.