The band will appear in new TV, poster and newspaper ads for Kit Kat Senses, which has just 165 calories and is aimed at women.
Details of the contract are reportedly still being finalised but it is believed to be their biggest advertising deal to date.
The girls -- Sarah Harding, Cheryl Cole, Nadine Coyle, Kimberley Walsh and Nicola Roberts -- also have promotional deals with Unilever hair care brand Sunsilk, as well as with Samsung Electronics.
Sunsilk signed a multimillion-pound one-year contract with the group last April, which covered a full marketing programme, including a TV and print ad campaign; on-pack promotion and in-store marketing; PR and endorsement; and sponsorship of the band's tour.
Samsung also signed Girls Aloud last year to market its music products through a series of promotional campaigns for mobile phones and MP3 players.
Separately, Harding has an existing deal with lingerie manufacturer Ultimo and Cole has fronted ad campaigns for Coke Zero.
Since being formed on the reality TV show 'Popstars the Rivals', Girls Aloud have notched up numerous consecutive top 10 singles, including 'Sound of the Underground', 'Love Machine' and 'Something Kinda Ooooh'.