"I don't wear tights," the Vermont Democrat said.
Leahy's scene was filmed this summer for "The Dark Knight" and involves Batman, played by Christian Bale, The Joker, played by Heath Ledger, and Alfred Pennyworth, played by Michael Caine.
The longtime Batman fan would reveal little about his role other than he is called the "distinguished gentleman."
"It's a pretty tense scene," said Leahy, chairman of the Senate Judiciary Committee. "It's going to be a very interesting one."
He's done voice-overs on Batman cartoons, written the preface for a Batman book and had small roles in the last two Batman features.
He said he will donate his earnings from the film to the Kellogg-Hubbard children's library in Montpelier, where the senator got his first library card.
"The Dark Knight" is scheduled to be released next summer.