Autotrader.co.uk will take a daily feed of What Car? news, road tests and features, which have been repackaged to fit in with the design of the web site. In return, What Car? will have its car-valuation service featured on the Auto Trader site.
The two web sites have traditionally been in competition for consumers, but the companies now see a natural fit between their brands.
"It would be unreasonable of us to imagine that consumers looking to buy their next vehicle will do all their research on one site," explained Martyn Hammond, sales and marketing director at Auto Trader.
"By knitting together our content with What Car?, we can provide a seamless service for car buyers, which will speed up their purchase decision and therefore encourage them to look to our site to buy vehicles," he continued.
"We chose to partner with What Car? as it is the leader in the provision of motoring editorial. Its strong association with the new car sector allows us to broaden the appeal of our site and demonstrate to the trade our commitment to provide vehicle buyers with the full spectrum of choice, new cars or used," added Hammond.
Kevin Madden, publisher of whatcar.com, said: "Our business model has always been to deliver an effective advertising medium for customers while offering the best motoring content to other businesses. This deal not only delivers a direct revenue from content supply, but it also offers us an opportunity to build advertising revenue via our valuation service."