The actress told the magazine that she thinks her breasts are too small to worry about going topless in her films.
© Mario Testino/Allure
"Sex scenes in films - I'm quite rigorous about what gets exposed... No bottom half!" she said.
"I don't mind exposing my tits because they're so small - people really aren't that interested!"
The star also talked down the controversial spanking scene with Michael Fassbender in David Cronenberg's A Dangerous Method.
© Mario Testino/Allure
Knightley said: "I was clothed most of the time. There was only once where you saw a nipple. It was quite extraordinary in the way how little was shown."
Of her topless advert for Chanel's Coco Mademoiselle in 2009, she added: "You see those breasts? They've got quite a lot of shape at the bottom!"
Comparing her breasts now to scenes in Pirates of the Caribbean: The Curse of the Black Pearl, Knightley said: "My back grew after that, and my tits disappeared. Isn't that annoying?
"When you get to 17, you think, Great! I've got tits! This is fantastic! And then all of a sudden you grow, and they disappear."
© Mario Testino/Allure
The actress complained about being given "really strange droopy tits" when her chest was digitally enhanced on the poster for
Allure editor-in-chief Linda Wells said of the new shoot: "We didn't do any surgery to Keira Knightley's breasts - neither with a scalpel nor a retoucher."
The full feature with Keira Knightley is in the December issue of Allure, on sale on November 20.
