"It's early, but they have talked about marriage," an insider told People magazine.
The source added that Madden has been spending a lot of time at the actress' Los Angeles home, so much, that "he kind of lives there" at the moment.
"They look very happy together...Cameron hasn't looked this happy for years," said the insider.
Diaz has previously dated video producer Carlos de la Torre, actors Matt Dillon and Jared Leto, Justin Timberlake and New York Yankee Alex Rodriguez. Madden, on the other hand, was engaged to Australian star Sophie Monk for a long time, before splitting up in 2008.