A series of banner ads will promote the service across Alta-Vista (www.altavista.co.uk).
Mark Nall, director of business development and sales at Alta-Vista UK, said: "This deal gives Autotrader highly qualified leads from our motoring section.
"Autotrader is paying us for the qualified traffic basically, but we are incentivised to perform well and the more traffic we put through, the better."
The deal follows growing activity within the online car market, including the recent launch of a £1 million motoring portal by Dennis Interactive, the multi-media arm of Dennis Publishing.
The portal, which is based on Dennis' Auto Express print title, will be backed by a £175,000 cross-media campaign.