Penn's camp declined to comment on the report, and Theron's rep did not immediately respond to the Los Angeles Times' request for comment.
The "Mad Max: Fury Road" actress called off the relationship after returning from the Cannes Film Festival in France, the reports said, and the twosome spent much of their time apart after the trip.
Engagement rumors, which buzzed last July and again last December, also came by way of Us Weekly, and the mag also said that Penn, 54, was moving to adopt Theron's son, Jackson, 3. However, neither actor's camp publicly commented on nor confirmed those reports, and People said it had learned no adoption was happening.
The longtime friends and activists, who each won acting Oscars in 2004, took their relationship from friendship to romance in December 2013 and made it red carpet official at the Fame and Philanthropy post-Oscar party in March.
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."
She said she "really didn't see it coming" and that their relationship came about naturally and effortlessly.
"Before I knew it, I was in something that was making my life better," she said.