With glittering gowns and high-octane frocks the standard attire for the glam French festival, the actress decided to eschew a dress in favour of trousers, instead.
Miss McAdams hit the Sleeping Beauty premiere in a golden bustier and high-waisted tailored trousers - both Monique Lhuillier - teamed with matching metallic cuff and clutch, and a bold red power pout.
The Cannes premiere also marked the first time that Rachel officially stepped out with new beau, Michael Sheen.
The look has prompted mixed results among fashion bloggers, with some criticising her for looking too casual, and others applauding her for doing something different.
Our thoughts? After her high-impact entrance the previous night, we reckon Rachel was conscious of not upstaging the Sleeping Beauty cast members, hence the more dialled down outfit. (Which really, is just being polite.)
Plus, we think she looks great.
Let us know what you think of Rachel's red carpet attire below!
See all the stars and all the style from the 2011 Cannes Film Festival here: