The schoolgirls were abducted April 15 by Boko Haram, and since then the girls' families have criticized the Nigerian government for failing to dedicate sufficient resources to find the missing girls.
The incident inspired a #bringbackourgirls Twitter campaign to which political figures and celebrities alike have added their support, and the latest is Hayek.
She held up a sign with the #bringbackourgirls hashtag while walking the red carpet for the premiere of the animated film The Prophet.
Marie-Pierre Hauville, director of communications for the film festival, said the move was "most unusual."
"It's rare. But clearly this is happening all over the world and was a personal impulse," she said.