The letter comes amid a debate over so-called cancel culture where prominent people face attack for sharing controversial opinions.
The forces of illiberalism are gaining strength throughout the world and have a powerful ally in Donald Trump, who represents a real threat to democracy, the letter said.
But resistance must not be allowed to harden into its own brand of dogma or coercionwhich right-wing demagogues are already exploiting.
In a series of tweets, Rowling said she supported transgender rights but did not believe in erasing the concept of biological sex.
The letter criticized the state of public debate and the swift and severe retribution dealt out to any perceived wrongs.