8:20 AM PT -- TMZ Sports has learned ... the NBA is currently looking into the situation.

Nikola Jokic's brother was in no mood to celebrate the Nuggets' comeback win over the Lakers on Monday ... 'cause he straight-up punched a fan in the stands at the playoff game -- and it's all on video.

According to witnesses, the violent incident went down shortly after Joker and his Denver teammates completed their 101-99 win against LeBron James and co. to go up 2-0 in the first-round series.

The clip shows Joker's older brother, Strahinja, hop over a Ball Arena seat and down a row ... kicking his way through the crowd to get closer to the male spectator.

As soon as Strahinja got within arms reach, he unloaded a fierce punch with his right hand that hit the fan right in the face.

The two sides weren't done with each other after the hit, though ... they continued to bark at each other as Strahinja's brother, Nemanja, followed right behind him for support.

The two-time MVP's wife, Natalija, was just feet away from Jokic's brothers throughout the altercation ... holding their young daughter as she reached out to reel Strahinja back.

It's unclear what led to the altercation ... we're working on it.

The massive Jokic brothers are notorious for passionately defending the NBA superstar at games ... and they're certainly not shy to let their feelings known to players, referees and fans.

The NBA is known to throw the hammer down on unruly fans ... but considering Jokic is one of the best players in the league, it'll be interesting to see if any punishment comes from this.