The British star looked breathtaking in a grey and silver Elie Saab gown with a feathered skirt and sequined top.
The "Insurgent" actress accentuated the dress' plunging neckline with an intricate silver necklace and her hair was pulled back into a loose bun.
But the frock looked even better when she turned around.
If we couldn't love her enough, Watts' stylist, Jeanann Williams, posted an adorable Instagram shot of her relaxing with little ones Sasha and Kai.
Looks like she needed a quiet moment after all the attention her dress probably garnered!
See more red carpet looks from the 2015 Cannes Film Festival here: