From the designer's Autumn/Winter 16 collection, the cheeky one-shouldered dress featured sheer silk skirt with blue and gold embellishment and a fishtail cut.
Rachel kept her accessories minimal but opted for an equally dramatic hair-do.
Appearing classic from the front, when the actress turned to the side she showed off a modern take on the pony tail.
Earlier this week, while promoting the film, Rachel revealed that she would be keen for a Mean Girls reunion.
“I loved that movie. I love Tina Fey. I love the girls," she told The Mirror.
"I mean, to get to kind of have a reunion would be...we got to have a reunion at a photo shoot a couple of years ago and it was over like that and it wasn't enough. I was back in it.”