Harry Potter star Emma, 20, walked away with the top award for Style Icon, which was presented to her by fashion designer Vivienne Westwood, and X Factor judge Cheryl triumphed in the Best Music category.
Emma has starred in campaigns for fashion house Burberry, and has designed two collections with ethical clothing label People Tree. She was also recently unveiled as the new face of Lancome cosmetics.
The actress said of her win: “It feels amazing. Fashion is something that I love. I try not to think about it too much. I try and block all that off and I just do what I love and just have fun with it.”
Elle editor Lorraine Candy praised Emma’s style, saying: “She’s got a very distinct, individual look. We always say at Elle, ‘It’s not what you wear, it’s the way you wear it’, and she’s got such spirit, Emma. I think that makes her stylish.”
Other winners on the night included supermodel Helena Christensen, who received the Outstanding Contribution to Fashion Award, and Swedish actress Noomi Rapace, star of The Girl With The Dragon Tattoo, who won the Breakthrough Talent title.
Christopher Kane was named Best British Designer and Tom Ford walked away with Best International Designer.
Alexa Chung, a previous winner of the top Style Icon award, hosted the bash.
In full: Elle style awards 2011 winners
Outstanding Contribution to Fashion: Helena Christensen
Best British Designer: Christopher Kane
Top International Designer: Tom Ford
Model Of The Year: Coco Rocha
Best Actress: Natalie Portman
Best TV Star: Blake Lively
Breakthrough Talent: Noomi Rapace
Style Icon: Emma Watson
Musician Of The Year: Cheryl Cole