The duo was photographed in Beverly Hills on Friday showing quite a bit of PDA and locking lips in the actress' car.
The South African beauty can be seen leaning in to Penn, who was driving, and cupping his face during the steamy passionate kiss.
A source tells E! News exclusively that it was just the two of them in the vehicle and when they paused at a stoplight, they "made out the entire time."
It's unclear where the Oscar winners were headed during their afternoon outing, but we do know one thing for sure: we have never seen them quite like this before.
This isn't the first time we have seen Penn and Theron show their affection for one another since they were first romantically linked just after the New Year.
The A-listers fueled relationship rumors when they spent the holidays in Hawaii together and were spotted returning to Los Angeles.
"They are spending night after night together and moving very fast," a source told E! News exclusively at the time of the sightings. "It seems like it's getting serious and that they can't get enough of each other."
The pair made their first official public appearance at Penn's Help Haiti Home gala on Jan. 11 and although they didn't walk the red carpet together, they were quite comfortable cuddling and giving each other sweet little kisses at their table.
Since then, the lovebirds have spent time with Theron's adorable 3-year-old son Jackson at the grocery store and at the beach. They also showed their first signs of PDA as they held hands during a romantic getaway in San Francisco, Calif.
—Additional reporting by Michelle Falls