The agency picked up the account following a pitch against DraftFCB, another Vauxhall roster agency.
It will be briefed with creating a launch campaign, involving web-build and advertising, for the second-generation Agila. The new model made its debut this week at the Frankfurt Motor Show and will be available in the spring.
The Agila is a mini people carrier, aimed at women. It will compete in the urban mini car market against the likes of the Fiat Panda.
In July, GM handed the £60 million pan-European creative launch brief to StrawberryFrog. The agency won the business following a pitch against McCann Erickson and the French agency Australie.
Digitas' most recent work for Vauxhall involved Flickr, the web-based photo-sharing tool, and saw users driving a Vauxhall through a landscape made of pictures from the site.