The 21-year-old actress, better known for her glamorous outfits and promoting brands like L'Oreal and Louis Vuitton, made the announcement at a party in New York last night.
Johansson, who also starred in Woody Allen's 'Match Point', will design her own retro-style range called Scarlett Hearts Rbk, which will be available next spring.
She said: "Reebok has a rich heritage in developing and designing cutting-edge women's product which makes them an ideal company to partner with."
Johansson, who was voted "Best dressed female celebrity" in a Yahoo! online poll earlier this year, will model the range herself in a global advertising campaign including TV, print, online and outdoor executions.
Paul Harrington, Reebok president and chief executive, said: "Reebok is thrilled to partner with Scarlett because she is a world-renowned style icon and truly an inspiration for today's young women.
"Scarlett embodies the pulse points of our brand -- individuality, authenticity and a life lived to the fullest in perpetual motion. These characteristics make her the perfect fit for our new women's footwear and apparel collection and also for our exciting new women's campaign."
If you have an opinion on this or any other issue raised on Brand Republic, join the debate in the .