The ELLE covergirl and Twilight star chose a heavily embellished, one-armed white Elie Saab minidress for the black carpet event.
After the premiere, K-Stew made a quick change into her off-duty uniform of jeans, a cotton vest and a comfy old hoodie.
Fans who camped out all week for a front row view at Los Angeles's Nokia Theatre greeted the film's stars with three hours of enthusiastic screams.
Dakota Fanning also chose a frock by the Lebanese designer, hers a slightly more subdued asymmetrical beige silk and lace number.
The boys cut dapper figures, with Robert "Edward Cullen" Pattinson donning a burgundy Gucci tuxedo, and werewolf Taylor Lautner rocking a tux in classic black, also from the Italian fashion house.
Ashley Greene and Bryce Dallas Howard upped the fashion stakes in floor-sweeping Alexis Mabille and Christian Dior, respectively. The best part of fame, according to Greene? Not the fashion, not the acclaim, but the ability to help worthy causes. "Look, I have a million followers on Twitter because of this movie, but what that really means is that if I tweet out that people should consider giving to donatemydress.com, all of a sudden that charity gets a response, and that amazes me. I couldn't be luckier," she told USAToday.More