That represented more than 3.8 billion adults and 746 million children and adolescents, research published in The Lancet said.
Obesity is associated with an increased risk of serious health problems like diabetes, some cancers, and heart disease.
"The unprecedented global epidemic of overweight and obesity is a profound tragedy and a monumental social failure," said lead author Professor Emmanuela Gakidou from the Institute for Health Metrics and Evaluation at the University of Washington in the US.
The team, part of the Global Burden of Disease Study BMI Collaborators, used data from 204 countries and territories to come up with their estimates and project what could happen in future without action.
Rates of overweight and obesity have already more than doubled over the past three decades, with more than 2.1 billion adults and 493 million young people aged 5-24 years affected as of 2021, they said.
While the reasons for obesity were complex, governments should use the estimates to focus in on how to help populations most at risk, researchers said, particularly around improving access to healthy diets.
The study, funded by the Gates Foundation, had limitations, including data gaps and the use of body mass index as a marker.
It also did not take into account the potential impact of obesity drugs, which have seen huge demand in richer countries where they are available, the researchers said.
The picture varies globally, but the researchers said an accelerated rise in obesity among young people and the rise in low- and middle-income countries where health systems cannot cope with the coming disease burden were particularly worrying.
That includes a projected 250 per cent increase in rates of overweight and obesity in sub-Saharan Africa. The rise, to 522 million adults and 200 million children and young people, is driven partly by population growth.
A separate study published on Monday, the World Obesity Atlas from the World Obesity Federation, also raised this issue.
"The most affected regions are developing countries," said Simon Barquera, president of the Federation.
The Obesity Atlas suggested that 79 per cent of adults and 88 per cent of children with obesity and overweight will be living in low- and middle-income countries by 2035, and only 7 per cent of all nations have adequate health systems in place to deal with this.
"It's really one of the main public health challenges around the world," Barquera added.
