The rare work by Rowling, over two sides of an A5 postcard, was stolen during a burglary in Kings Heath in south Birmingham.
On a post on Twitter, Rowling retweeted the police’s appeal and said: “Please don’t buy this if you’re offered it. [It was] originally auctioned for [the charity] English Pen. The owner supported writers' freedoms by bidding for it.”
English Pen is a charity promoting the freedom of expression.
Investigating officer PC Paul Jauncey said: "The only people who will buy this unique piece are true Harry Potter fans. We are appealing to anyone who sees, or is offered this item for sale to contact the police."
A number of members of the book trade also appealed to their followers to look out for the stolen postcard.
Friends and colleagues, please look out for this item and contact the West Midlands Police if you see it being offered https://t.co/siamdnd1f4— Peter Harrington (@PHrarebooks) May 12, 2017
A quantity of jewellery was also stolen from the address between April 13-24.
Anyone with information about the break-in are asked to contact West Midlands Police on 101, quoting crime number 20BW/83503C/17.