Mr Reagan's doctors said the 83-year-old ex-president was entering the early stages of the progressive brain disease.
``I have recently been told that I am one of the millions of Americans who will be afflicted with Alzheimer's disease,'' Mr Reagan said in a handwritten letter to the American people from his Los Angeles office.
Mr Reagan, who served two terms from 1981 to 1988, said he decided to disclose the diagnosis to focus public attention on a disease that mostly afflicts the elderly.
``In opening our hearts, we hope this might promote greater awareness of this condition,'' he said.
``Perhaps it will encourage a clearer understanding of the individuals and families who are affected by it.
``At this moment, I feel just fine. I intend to live the remainder of the years God gives me on this earth doing the things I have always done,'' he said.
Alzheimer's disease, or pre-senile dementia, is degenerative, attacking the cerebral cortex. It shows first in failing memory.
The precise cause of the disease is not known and no treatment has been found.