"I'm so disappointed by this. This is literally the entire conservative bathroom argument put into a novel," posted one user.
"I have been giving Rowling the benefit of the doubt regarding her questionable trans comments and then she goes and writes this tone deaf load of bull," another said.
A former fan said: "You used to be one of my favourite authors. My daughter and I used to argue over whether we can separate the art from the artist. Now we agree that we can't."
"Oh. So the transphobic tweets in recent months weren't random but publicity for the new book," another tweeted.
Rowling has repeatedly shared her controversial views on her Twitter account, prompting Harry Potter stars, including Daniel Radcliffe, to publically condemn her comments. She also penned a lengthy essay detailing "her reasons for speaking out on sex and gender issues", which was widely criticised for its inaccurate arguments.
Trans activists and celebrities denounced her arguments, including UK charity Mermaids and American organisation GLAAD.