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.