The comedy actor, who played Nearly Headless Nick in Rowling’s franchise, said he had added his name to a list of 58 public figures supporting Rowling.
Writing on Twitter on Wednesday, Cleese said: “Dear Twits, I have added my name to the signatories of the letter in solidarity with JK Rowling.
“Proud to be in the distinguished company of Ian McEwan, Andrew Davies, Frances Welch, Lionel Shriver, Ben Miller, Tom Stoppard, Frances Barber, Griff Rhys-Jones and Matthew d’Ancona.”
When one follower questioned his support, telling him he was taking a “huge step backwards” that would allow the author the “continue hurting others”, Cleese shrugged off the criticism.
“I suspect that the signatories I am proud to be associated are far more balanced and intelligent than the people whose slogans you chant. Sad”, the 80-year-old replied.
Addressing another disappointed fan, Cleese wrote: “People who feel themselves specially ‘good’ often achieve this feeling of purity by using the denial and projection defence, seeing their own hatred in others. I see no hatred whatsoever among the signatories of the JKR letter. Perhaps it exists elsewhere, among the deranged”.
The letter condemns what it calls the “insidious, authoritarian and misogynistic” opposition to Rowling on social media.
It claims that Rowling “has consistently shown herself to be an honourable and compassionate person”.
The letter purports to stand up to “hate speech directed against her and other women”.
The signatories wrote: “We are signing this letter in the hope that, if more people stand up against the targeting of women online, we might at least make it less acceptable to engage in it or profit from it”.