But are they really? We'll get to that in a sec.
A rep for Penn declined to comment on any of the rumors, only saying that the latest story was "all speculation." Reps for Theron did not respond to requests for confirmation.
Meanwhile, the gossips churned out details, saying that there was no diamond engagement ring because the low-key actress doesn't care for the all the trappings of traditional proposals.
"But they are committed," a source told the mag.
The latest she-said-yes report comes just a day after E! News reported that the couple was getting even more serious, though they weren't engaged per se.
"They have made a commitment to be together for the rest of their lives," a source told the outlet. "They are discussing when and where they will have a wedding."
The actress, 39, addressed the high-profile coupling head-on in the June issue of Vogue after months of being spotted on the actor-director's arm. The forthright star said she didn't "know how you have a healthy relationship with the whole world knowing about it."
"What I guess I am trying to say is: A friend kind of stepped into that role [of making my life better], and I really didn't see it coming," she said. "Sean and I have known each other for 18 years. We were just really good friends. And it’s been slow going because you are aware that you could screw that up and lose it. We had to think about it. But at the end of the day, it was somewhat effortless. It just kind of naturally happened, and before I knew it, I was in something that was making my life better — the people who really love me can see the effect it has had on me."
Penn, 54, a two-time Oscar winner, is said to have hit it off with Theron's son Jackson, whom she adopted in 2012.
The Dior spokeswoman is starring in Penn's latest directorial effort, "The Last Face," which they shot in Theron's native South Africa in the fall. She costars with Javier Bardem and together they play humanitarian workers who meet while providing aid to unrest-stricken regions.
If the actors are indeed engaged, this would be the third marriage for Penn, who was previously wed to music icon Madonna from 1985 to 1989 and to "House of Cards" star Robin Wright from 1996 to 2010. He and Wright have two adult children together, Dylan and Hopper.
Theron was previously in a nine-year relationship with Irish actor Stuart Townsend that ended in 2010.
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