"Only last year, we saw an online petition to ban Donald Trump from entry in the UK. It garnered half a million signatures," she said at Pen America's annual literary gala here.
"Now, I find almost everything that Mr. Trump says objectionable. I consider him offensive and bigoted. But he has my full support to come to my country and be offensive and bigoted there," she said.
As a result of the petition, the British parliament held a debate but no vote in January, with lawmakers using the discussion to air their views on Trump under the protection of parliamentary privilege, which legally shields members from accusations of defamation or slander.
Rowling, 50, who accepted an award at the Pen gala for her humanitarian work, said the freedom that protects Trump's right to speak his mind also protects her right to "call him a bigot."
Last December, after Trump stood by his proposal for a temporary ban on Muslims from entering the US, Rowling tweeted, "How horrible. Voldemort was nowhere near as bad.