The 'Devil Wears Prada' actress will star in the new TV and print campaigns for the "evocative" scent - which was created by perfumer Jean-Louis Sieuzac in 1977 - and Emily revealed she loves fragrances because scent is so important to her.
She explained to People: "I think really that's what I love most about it - it evokes so much for people. Everyone always says they've smelled something, and remember this beautiful memory they experienced maybe many years ago. There's something very powerful about it."
Emily's new advertising campaign for the brand - where she plays a woman on the hunt for the Opium fragrance - will air in December and Emily revealed she was thrilled to play a role rather than simply pose with the product.
She said: "It was thrilling. I was given a story to play, and for me as an actress that's very exciting. I'm not just posing or just trying to sell something."
Speaking about the perfume previously she said: ""It is a really exciting experience to embody this legendary fragrance, which has stood the test of time without its allure ever being diminished. Being a part of this new campaign and being able to tell the story of this very evocative fragrance is a real thrill."
Renaud de Lesquen, president of YSL & Designer Brands, explained the new campaign will portray "a fiery, almost reckless woman with magnetic seduction, that nobody or nothing can resist".
He added: "Opium embraces the whole history and values of our house."