By 2050, more than 153 million people could have dementia, up from 57 million in 2019, experts are warning.
The predicted rise is largely down to ageing and growing populations.
But unhealthy lifestyles contribute too, the researchers say in The Lancet Public Health journal.
Risk factors that urgently need addressing and account for more than six million of the projected increase include high rates of smoking, obesity and diabetes, they say.
The study, which looks at 195 countries, aims to give governments an idea of what resources and support may be needed and what action might help.
Dementia is already the seventh leading cause of death worldwide and one of the major causes of disability and dependency among older people.
But it is not an inevitability. The researchers point to the importance of improvements in access to education in countries around the world and say that their projected figure for 2050 has already been adjusted downwards by 6.2 million based on what is expected to happen in this area.
They are less optimistic about the effects of obesity, high blood sugar and smoking and have already factored in an extra seven million cases in 2050 linked to those causes.
Lead author Emma Nichols, from the Institute for Health Metrics and Evaluation, at the University of Washington, in the US, said: “We need to focus more on prevention and control of risk factors before they result in dementia.
“Even modest advances in preventing dementia or delaying its progression would pay remarkable dividends.
“To have the greatest impact, we need to reduce exposure to the leading risk factors in each country.
“For most, this means scaling up locally appropriate, low-cost programmes that support healthier diets, more exercise, quitting smoking and better access to education.”
The study predicts cases will rise:
- In eastern sub-Saharan Africa, from nearly 660,000 to more than three million – mainly driven by population growth
- In North Africa and the Middle East, from almost three million to nearly 14 million
- In the higher-income Asia Pacific region, from 4·8 million to 7·4 million
- In Western Europe, from almost eight million to nearly 14 million
- In the UK, from just over 907,000 to almost 1.6 million