Coleen, whose husband Wayne is currently playing for England in the World Cup, will front a £4.5 million TV advertising campaign during the autumn.
Speaking about the deal, the mother-of-one said: "The role is a perfect match, enabling me to select and wear my favourite styles as well as share my ideas on how to carry off the season's new trends."
Littlewoods' Shop Direct retail director Gareth Jones commented: "Coleen has great style, a real interest in fashion and is a very approachable and warm personality. Not only do our customers identify strongly with her, they also have huge admiration for her work championing fashion and style for real women."
Coleen's style ambassador role will include offering fashion tips to customers in the store's directory.