The new couple, who have apparently been dating for about a month, are said to have been introduced by Nicole Richie, who is married to Benji's brother Joel, reported Hello! magazine.
Diaz, 41, and Richie have recently been snapped hanging out together and growing closer. Earlier this week, the Hollywood beauties attended Vicky Vlachonis' The Body Doesn't Lie launch party in Los Angeles, alongside celebrity friends Gwyneth Paltrow and Chris Martin.
The former Charlie's Angels star seemed eager to keep her new romance under wraps, as she posed next to author Vicky for a photo, with her rock star boyfriend standing nearby.
"There are many soul mates. My soul has a lot of different facets and it needs a lot of different men. Friends, too. Friends can be soul mates," she had said.