The number of people in China with the syndrome has more than doubled over the past 20 years, a team led by University of Edinburgh scientists said.
Rates of dementia in the country are increasing faster than previously thought and dealing with the problem will soon cost health services more than tackling heart disease and cancer combined, the researchers warned.
The health records of almost 350,000 people in China were looked at to see how many were diagnosed with dementia, including Alzheimer’s disease, between 1990 and 2010.
In 1990, 3.68 million people had dementia, rising to 9.2 million by 2010.
Increased life expectancy in China and demographic, social, economic and lifestyle change over the period may help explain the rise, the study said.
Estimates by the World Health Organisation and other bodies may have underestimated the burden of chronic diseases and conditions such as dementia in the developing world, the scientists warned.
Rapid government action is needed to tackle the problem, they said.
Dr Kit Yee Chan, lead author of the study at the University of Edinburgh, said: “Of the many chronic diseases that need attention worldwide, dementia is predicted to have the greatest economic and social effect.
“The number of dementia and Alzheimer’s cases in China might pose the single largest challenge to health and social care systems in terms of finding appropriate and affordable responses.”