Will Smith Surprises Fans at 'Bad Boys 4' Showing on Release Day

Will Smith totally stunned fans of his hit 'Bad Boys' franchise Friday ... dropping in at a random L.A.-area screening.

The actor shared a clip to his Instagram, where he hits a theater in Baldwin Hills -- a neighborhood in Los Angeles -- donning a mask to keep a low profile while joining the crowd at "Bad Boys: Ride or Die" which was released wide yesterday.

Smith says it's a tradition for him ... whenever he has a new flick coming out, he goes to a few different showings of the flick -- one on Friday, Saturday and Sunday.

The clip then takes fans inside the theater where they can see moviegoers laughing and clapping during the huge summer blockbuster starring Smith and Martin Lawrence in their iconic roles.

Seems like fans really enjoyed the action-comedy ... but not as much as some fans enjoyed the aftermath -- where WS threw off the mask and asked some fans leaving if they liked the movie.

Check out the vid ... always a man of the people, Will takes pictures and videos with his legions of fans who compliment the dude on his acting chops -- and the whole group takes one big selfie together too.

Will's got a ton to be happy about it seems ... 'cause 'BB4' made more than $21 million on opening day, and projections have it clearing up this weekend with a solid $53 million opening.