The actress looked stunning in a curve-clinging Safiyaa dress, which she paired with Christian Louboutin shoes and an Alexander McQueen bag.
The cute capped sleeves and clean simple lines were perfect on her figure, and nobody would be able to tell she welcomed son Bear just three months ago (What?!).
Kate finished the look with tousled curls and red lips, as well as a cool gold cuff, which we would actually like in our wardrobe now.
The yummy mummy was joined at the bash by co-stars Ashley Judd and Shailene Woodley, who had a golden moment in a shimmering one-shouldered Elie Saab Couture dress.
Shailene has recently been singing the virtues of eating clay (yes, you read that right), as apparently it flushes your system naturally giving you a healthy glow. If it really does make you look like this, perhaps we might give it a go, after all. OK, probably not, but whatever she's eating she does look fab.
The new film, directed by Neil Burger, sees Shailene play Tris, who learns she's 'Divergent' and won't fit in to a world divided by factions based on virtues.
When she discovers a plot to destroy Divergents, Tris and the mysterious Four must find out what makes Divergents dangerous before it's too late.
MORE KATE WINSLET? Flick through her style file below: