Charlize Theron surprised a boxing champ on his birthday -- threw punches at him and then kissed his face ... and we've got footage of the whole thing!
The man on the receiving end is Chris van Heerden -- a former IBO Welterweight champ who's a BIG fan of the actress (they're both from South Africa).
Peter Berg -- the mega-producer who owns the gym -- wanted to surprise Chris on his birthday so he got Charlize to swing by for a sparring session ... and Chris LOVED IT, Peter Kahn, van Heerden's manager tells us.
Charlize not only got in the ring and threw hands -- she jumped on the fighter, wrapped her legs around the guy ... and capped it off with a big, fat kiss on his face.
Oh, and Mark Wahlberg also stopped by. Best birthday ever?!
By the way, Theron's not a regular at the gym ... but she wasn't bad on the mitts for an amateur.