Her show-stopping, mermaid-style Marchesa gown featured intricate embroidered details, and a voluminous tulle train that went on for miles.
The Desperate Housewife won the prize for standout gown at the premiere of Moonrise Kingdom.
The 38-year-old was closely followed in the style stakes by Diane Kruger, who looked ethereal in a one shoulder Giambattista Valli Couture gown.
She floated up the red carpet, trailing a long cape-like swathe of aqua silk behind her.
Other beauties at the event included Freida Pinto, Jane Fonda and Eva Herzigova.
After the celebrities had checked out Wes Anderson's romantic comedy – which tells the tell of two young lovers who cause havoc when they run away – they moved on to the opening ceremony dinner.
Eva changed into a gorgeous monochrome dress by Alberta Ferretti for the bash, at which Beth Ditto sang.
Freida also debuted a second outfit – she swapped her funky pink Michael Angel number for a glittering black sequin Versace dress.
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Glamour through the ages as Eva joins Freida and Jane on the Riviera