The Mystic River star was enjoying a drink with a female companion at the Shade Hotel in Manhattan Beach on Thursday (26Jan12) when an intoxicated customer began shouting at the barman, interrupting other patrons' evening out.
An annoyed Penn approached the man to ask him to stop his rant, according to EOnline.
An onlooker tells the website, "This guy was drunk and yelling at the bartender. He wasn't making much sense, but he was loud and just kept screaming.
"Sean came over to the bar and said, 'Is everything OK over here?' The guy was a smart alec and said to Sean, 'Stay out of it, Sparky!'"
The website reports the man left the bar after his run-in with Penn.