A spokesman for the Beauty and the Beast star confirmed the pictures - which show her trying on various outfits - were stolen.
"Photos from a clothes fitting Emma had with a stylist a couple of years ago have been stolen," her publicist said.
"They are not nude photographs. Lawyers have been instructed and we are not commenting further."
News of the theft comes a week after Watson sparked fierce debate over whether she was anti-feminist for exposing part of her breasts in a Vanity Fair photoshoot.
The actress was previously threatened in 2014 with a leak of nude images, after a speech she gave on gender equality as a UN Ambassador for Women.
Speaking at a Facebook event in 2015, she said: "I knew it was a hoax, I knew the pictures didn't exist.
"The minute I stepped up and talked about women's rights I was immediately threatened - within less than 12 hours I was receiving threats."
Last month, a man in the US was jailed for nine months for hacking into the online accounts of celebrities, including Jennifer Lawrence, and stealing private information, including nude videos and photos.
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Emma Watson is taking legal action after dozens of private images of her appeared online.