The British actress wore a pink lace design with metallic embroidered detailing and silver block heels. The barely there make up and that layered mid bob? She's so boho glam.
The dress was breathtaking and the Brit actress proved that when it comes to kicking off party season style - she's a pro. This is how to do elegance (floor-sweeping with a statement heel) and edgy (rock chic hair) at the same time.
Man, we just love her tousled blonde crop.
Sienna, 32, arrived on the arm of her boyfriend Tom Sturridge, 28, who was also looking pretty dapper in top-to-toe Burberry.
While Sienna's look turned heads all evening, Naomi Campbell was also channeling party season glamour in a plunging gold sequin gown by Burberry complete with thigh-high split and matching stilettos (only a legendary model like Naomi can do cleavage and legs).
Even Victoria Beckham was at the ceremony looking chic in a signature black, strapless gown from her own collection which she accessorised with a statement sapphire ring and drop diamond earrings.
If this ensemble looks a little familiar, it's because she wore the same dress in white to the 2014 Met Gala.
LOVE SIENNA'S STYLE? Take a look through this gallery to see more looks from the blonde bombshell.