The dress, by French fashion house Saint Laurent, featured gorgeous cut out detail and a built-in chain necklace.
The star teamed her stunning dress with sky high Saint Laurent “Janis” pumps, a Hermés cuff bracelet and dainty silver rings.
The 34-year-old actress went for a retro look with her hair and makeup with a side-swept beehive and girly frosted pink lipstick.
This outfit is the second super stylish look from McAdams in the last week with her futuristic Roksanda Ilincic dress also impressing at the London premiere.
If you’re loving Rachel’s look as much as we are, you can get a similar version of her LBD from River Island and Asos.