Will Smith Crashes J Balvin's Coachella Set to Perform 'Men in Black'

Will Smith made a rare appearance onstage this weekend -- crashing Coachella to perform a song from the '90s ... and doing it alongside none other than J Balvin.

Indeed ... very random, but no less entertaining. Balvin had an alien-themed design and aesthetic for his Coachella gig ... and at one point in the set, he brought out WS who was rocking his signature 'MIB' 'fit ... only to rap the "Men in Black" song from the titular movie.

The crowd loved it ... including Jada Pinkett Smith, J's girlfriend, Valentina Ferrer, and BLACKPINK sensation Lisa who were seen dancing together in the artist pit. Will's performance is getting rave reviews too, 'cause people could clearly hear his vocals as opposed to a backing track.

As for why Balvin brought out Will at all ... anyone's guess, but hey -- it was cool. We know Will's got a new 'Bad Boys' movie to promote, but this really doesn't serve that purpose.

The Smiths were at Coachella as a family, it seems, 'cause Jada Pinkett-Smith was there too ... as was Jaden Smith, whom we saw hanging out with Justin Bieber this weekend.  Their daughter Willow was performing at the festival ... so makes sense the whole crew was there.

No onstage antics from Will this time around ... dude was on his best behavior here.