The one-year contract covers a full marketing programme, to include a TV and print ad campaign from June 1; on-pack promotion and in-store marketing; PR and endorsement; and sponsorship of the Girls Aloud tour in May.
JWT London will be producing the new creative, which features each girl promoting a different variant of shampoo and conditioner suitable for their hair type. The ads will be in the same vein as Sunsilk's previous outings, which feature different shampoos suitable for each hair type.
A Sunsilk spokesperson said: "This is the first time the girls have collectively worked with a brand and the first time Sunsilk has featured celebrities in its advertising so we're hugely excited about the deal."
The official launch of the campaign will be on April 12 in central London, where the girls will be doing a photoshoot and giving interviews regarding the deal.
Band member Sarah Harding recently launched Ultimo's latest lingerie range, following in the footsteps of supermodels Helena Christensen and Rachel Hunter.
Coca-Cola chose Cheryl Cole, then Tweedy, to front the launch of Coke Zero -- a sugar-free drink, dubbed "bloke coke" in July last year. The product is aimed at young adult males, who do not traditionally buy diet drinks.