"I would have liked to keep this one quiet for a little while," he wrote in a letter headed 'An Embuggerance'.
"But because of upcoming conventions and of course the need to keep my publishers informed, it seems to me unfair to withhold the news."
The author said work was continuing on his latest works, Nation and Unseen Academicals, and that there was "time for at least a few more books yet".
"All other things being equal, I expect to meet most current and, as far as possible, future commitments."
"Frankly, I would prefer it if people kept things cheerful," he continued, saying it was "too soon to tell" if the condition was immediately life-threatening.
"I will, of course, be dead at some future point, as will everybody else. For me, this maybe further off than you think.
"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."
Apparently Pratchett has decided to help those experts along in their studies, by donating $1m to Alzheimer's research.
"It is a shock to find out that funding for Alzheimer's research is just 3% of that to find cancer cures."
In total, an estimated 700,000 people in the UK have Alzheimer's disease.
However, the Alzheimer's Research Trust estimates that just £11 per patient is spent annually on research into the disease - compared with £289 for each cancer patient.
I can't say I blame him.