The business, which includes online advertising and website creation, was previously held by Syzygy. The agency has been charged with increasing the brand's profile online and driving sales.
Paul Alexander, the business director at Rapier, said: "We are very excited about extending our work into the digital space. Online is now used twice as much as specialist magazines by car buyers to compare prices and features. Our role is to drive traffic to the site and to convert interested prospects into test- drives and ultimately sales."
Sam Bridger, the head of marketing at Smart, said: "Rapier's excellent understanding of our strategic and brand objectives and its focus on measurement and effectiveness will help drive greater efficiencies and maximise the growing online opportunity."
Rapier started working on Smart in 2003, when it won the direct marketing business in a five-way pitch.
In 2004, the number of Smart car registrations rose by 21.6 per cent to 11,844.
The agency handles retention and acquisition of customers including the conversion of prospects and encouraging repeat purchase from existing Smart car owners.