The former Gossip Girl star, who is married to Ryan Reynolds, showed off her model-like frame in black, lingerie-inspired slip dress by Marchesa as she attended the Van Cleef & Arpels store launch in New York.
The designer gown featured delicate lace straps and sheer panels.
The star, 26, wore her hair in sleek, side-swept waves and added a berry-hued lipstick. She accessorised with stunning diamond drop earrings, a diamond bracelet and black sequin two-strap sandals.
She was joined by Dita Von Teese at the event, who dazzled in a floor-length silver dress.