"If I had known this would have made you so happy, I would have told you years ago," Rowling told a full house of fans at New York's Carnegie Hall Friday.
Rowling said she slipped movie director David Yates a note about Dumbledore's sexuality after she noticed a reference to a girl in Dumbledore's past during the making of the sixth Potter film, Harry Potter and the Half-Blood Prince, E! Entertainment reported.
Rowling, on her first U.S. book tour since 2000, said love briefly blinded Dumbledore to the evil of Gellert Grindelwald, who became a master of the dark arts in the Potter books.
"Falling in love can blind us to an extent," Rowling said, adding that after Dumbledore was "horribly, terribly let down" he went on to destroy Grindelwald in a classic wizard battle between good and evil.