Researchers say the discovery is exciting because it may show what causes the disorder and ways to prevent it.
The problem gene is not common - less than 1 percent of people are thought to have it. However, it about triples the chances of developing Alzheimer's compared to people with the normal version of the gene. It also appears to harm memory and thinking in older people without dementia.
The gene helps the immune system control inflammation in the brain and clears sticky deposits that are part of the disease. Mutations in the gene may impair these tasks, so treatments to restore the gene's function and quell inflammation may help.
The new gene, called TREM2, has already been tied to other forms of dementia. Further tests on Alzheimer's patients and others without dementia found that the gene variant was more common in Alzheimer's patients.
This finding is described in a study by an international group published online Wednesday by the New England Journal of Medicine.
About 5 million people in the U.S. have Alzheimer's. About 35 million people worldwide have dementia, and Alzheimer's is the most common type.
Medicines such as Aricept and Namenda just temporarily ease symptoms of Alzheimer's. There is no known cure.
The Associated Press contributed to this report.