WILL SMITH attended the Moscow premiere of "Men In Black 3" the other day.  And during his stroll down the red carpet, he was confronted by a reporter who was a little too "kissy".  A MALE reporter.

--The guy got slightly aggressive with his lips.  He stole a kiss on each of Will's cheeks, but it seemed like he was really trying to zero in on Will's lips.

--So Will shoved the guy back and then BACKHANDED him across the cheek, saying, quote, "Come on, man, what the hell is your problem, buddy?"

--To be fair, it wasn't a HARD slap.  And Will was kind of laughing as he told another reporter, quote, "He tried to kiss me on the mouth.  He's lucky I ain't sucker punch him."

--A witness says, quote, "He was still a good sport about it and thought it was funny after the fact."