Gennady Golovkin: Opponent Beat Up Woman ... Before Fighting GGG

Gennady Golovkin dished out karmic justice when he KO'd Dominic Wade back in April -- 'cause Wade had beaten the hell out of a woman in the months before the fight ... TMZ Sports has learned.

Triple G annihilated Wade during their April 23rd fight -- but it's shocking Wade was even allowed in the ring considering he was facing some pretty disturbing domestic violence allegations at the time.

According to official records, obtained by TMZ Sports, Wade was charged with assault for a Dec. 23rd incident -- in which he allegedly attacked the mother of his child ... and then punched her niece in the face.

The niece told cops she had heard the commotion in the home and ran into the room to help her aunt when Wade socked her in the mouth -- knocking out one of her teeth.

The case went under the radar before his big fight with GGG -- but weeks later, he pled GUILTY to one count of assault with significant bodily injury.

He's facing up to 3 years in prison when he shows up for his sentencing hearing in August.