Nikola Jokic's Brother Punches Fan In Heated Altercation, NBA Investigating

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 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.