Nikola Jokic's brother was in no mood to celebrate the Nuggets' comeback win over the Lakers on Tuesday ... '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 .