X17online got exclusive pics of Lindsay Lohan partying at Paris Hilton's Malibu beach house on Sunday, and we caught her hanging out with ... and hanging on to ... a mystery man.

Well, mystery solved: the guy is Australian actor Brett Tucker, 39. He's a paltry 14 years older than Lohan, 25.

Lindsay spent the day smoking and partying at Hilton's home, then left with Tucker. She held on to him as the two walked out, including putting her hands on his shoulders (above.)

Tucker is reportedly quite the ladies man, though he's had a hard time making it big in the U.S. -- the actor has only been able to land bit parts on TV shows like CSI and the now-canceled ABC series Off the Map. This will certainly raise his profile!

Of course, Lindsay might not be content with just one man ... read who else she's been getting close with after the jump ...