The actress put on a rather leggy display with a sheer lavender gown with ruffles that flowed all the way down to the Cannes red carpet.
Underneath the dress, Sienna wore a plunging playsuit to reveal a glimpse of her long legs and slim figure.
Her silver heels also gave her a few inches of height next to her tall and slim boyfriend Oli, who sharp black suit and tie and a pair of shades.
She completed the look with a sparkling diamond necklace, adding an appropriately glamorous final flourish for the prestigious occasion.
Her daughter, meanwhile, wore an adorable pale pink dress with an oversized bow.
Marlowe was the spitting image of her famous mother as they hugged and held hands for photos on the way to the film.
Sienna and co. were later joined by Horizon’s director and star, Yellowstone actor Kevin, who was rocking a surprising new look.
Having become even more iconic as John Dutton, a clean-shaven ranch-owner hiding dirty secrets, Kevin is now sporting a thick moustache and soul patch.
His epic new project Horizon will be releasing its first chapter in cinemas on June 28 in the United States, with part two coming August 16.
