Pratchett said the donation would go to the Alzheimer's Research Trust, a British charity.
"There's nearly as many of us as there are cancer sufferers, and it looks as if the number of people with the disease will double within a generation," the writer told the charity's annual conference.
He said Alzheimer's was "a nasty disease, surrounded by shadows and small, largely unseen tragedies."
Pratchett is best known for his Discworld fantasy series. More than 55 million copies of his books have been sold worldwide.
The writer said he had reacted to his diagnosis with "a sense of loss and abandonment (and a) violently coherent fury."
"That fire still burns," Pratchett said, but reassured fans that he plans to continue writing.
His latest work, "Making Money," was published in September and Harper Children's was due to publish a non-Discworld novel, "Nation," in 2008.
