Customers who spend more than £1000 on home furnishings will be entered into a draw to win one of 20 Smart Cabrio cars.
The embattled retailer sees the promotion, organised by Mediator, as offering added value to customer visits.
M&S chose the promotion ahead of a discounting option to protect what it claims is the premium brand image of its homeware.
DaimlerChrysler Retail, the dealerships group, is using the tie-up to achieve city-centre exposure. It aims to tackle people's scepticism about the small size of the Smart car and draw them into testdrives at out-of-town dealerships.
Smart brand manager Matt Wrigley said the tie-up was an efficient way of getting more out of the dealers' limited marketing budgets, which total just £700,000.