Will Smith ATTACKS Kissy Reporter

Will Smith just slapped a Ukrainian reporter across the face at the Moscow premiere of "Men in Black 3" -- and it was all caught on tape.

The Ukrainian reporter tries to kiss Will on the lips on the red carpet and the actor angrily pushes him away and then backhands him across the face.

You can clearly see that Will is incredibly pissed that the guy showed overt affection toward him.

After Will slaps the guy, Will says, "He's lucky I didn't sucker punch him."

We're told the dude who kissed Will is a television reporter who often kisses celebrities -- it's his schtick.

For the record, Will just came out in support of gay marriage.

A source on scene tells TMZ, the reporter's mouth brushed up against Will's and that's why the actor reacted so violently. Despite the incident, Will stayed positive ... and happily continued to sign autographs for fans and do red carpet interviews.