NBA Free Agent -- TASED IN THE ASS ... During Insane Bar Brawl

An NBA free agent was tased in the ass cheek outside a Miami nightclub this weekend ... after getting into a crazy brawl with an NFL player ... this according to the police report obtained by TMZ Sports.

Cops say they arrived at Dream nightclub around 5:20 AM Sunday morning -- and noticed St. Louis Rams linebacker Jo-Lonn Dunbar in a "heated verbal altercation" with NBA free agent Donte Greene.

According to the report, security was "unable to physically" restrain the men and their respective entourages ... and a fistfight ensued.

Cops say the situation escalated -- with "multiple subjects exchanging blows" ... and eventually they saw Greene approach Dunbar from behind and punch him in the head and neck with closed fists.

Cops say they warned Greene to stop but he didn't -- so they broke out the taser and fired two prongs into the 6'11" athlete ... connecting with his lower back and his right butt cheek.

Once cops activated the shock, Greene fell on his back -- and the fighting ceased.

He was eventually arrested and while in custody, cops say he apologized for his actions -- explaining that Dunbar had "been after me for years since I got into the NBA. He was beating my brother and I had to do what I did."