From the sweet bow detailing to the layers of chiffon making up the skirt, this look is not for the feint hearted, and Sienna absolutely nails it.
Keeping the look relaxed and ethereal, the star wore her blonde hair soft and wavy, but did add a red lip for a bit of edge.
Sienna was joined at the premiere by co-star Ben Affleck who also stars in the film which is set in the Prohibition era in the world of organised crime.
Speaking of the pair's intense sex scenes, the actress said she found it hard to keep a straight face.
"Ben and I are like brother and sister, thank God, so there was no awkwardness. There was just a lot of stupid giggling. He's very professional—I am not, but he is," she told Daily Mail.
"In that environment, it was a cool scene. There was a montage in the script saying we did it everywhere: in the car, in the bar...I was like: 'That's an entire day of just love scenes! OK. How do we do this?'"
"Obviously, by the time nine hours of it has gone past, I was shaking with tears running down my face. I mean, I can't tell you—but you have to laugh. Ben is just professional. It is what it is."