Discworld author Pratchett has sold more than 55 million books worldwide.
The writer, 59, who gave the news on the website of Discworld artist Paul Kidby, said the condition was behind a "phantom stroke" earlier this year.
He said: "I would have liked to keep this one quiet for a little while, but because of upcoming conventions and of course the need to keep my publishers informed, it seems to me unfair to withhold the news.
"I have been diagnosed with a very rare form of early onset Alzheimer's, which lay behind this year's 'phantom stroke'."
He added: "We are taking it fairly philosophically down here and possibly with a mild optimism.
The author said work was continuing on his latest book, Unseen Academicals.
... and he succeeds in keeping me smiling even with this sad news...
"All other things being equal, I expect to meet most current and, as far as possible, future commitments but will discuss things with the various organisers," he said.
"Frankly, I would prefer it if people kept things cheerful, because I think there's time for at least a few more books yet."
He told fans the statement should be interpreted as "I am not dead"."I will, of course, be dead at some future point, as will everybody else," he said.
"For me, this may be further off than you think. It's too soon to tell."I know it's a very human thing to say 'is there anything I can do', but in this case I would only entertain offers from very high-end experts in brain chemistry."
Pratchett was made an OBE in 1998 for services to British literature.
