Megan Thee Stallion appeared to accuse her ex-boyfriend, Pardison Fontaine, of cheating in a recent track of hers -- and now, the guy's firing back ... with his own diss song.

The rapper/writer dropped a new song called "THEE PERSON" ... and it's obviously aimed at Meg -- with the lyrics explicitly mentioning her by name, plus a ton of other jabs at her character that he says he's observed in the aftermath of their relationship.

You can check out the song for yourself ... but basically, he suggests Meg's somewhat of a troubled person -- at least as he knew her -- saying she should look at herself in the mirror.

Pardi appears to reference MTS's time with Tory Lanez -- before the 2020 incident -- as well as other guys he heavily implies she might've slept with ... despite her telling him she hadn't to his face. In other words, Pardi's getting into some alleged bedroom tea of his own.

Of course, this comes after Meg dropped her single, "Cobra," just a couple weeks ago -- where she claimed she'd caught Pardi creeping with another woman while they were together.

They quietly broke up earlier this year ... with Meg recently hanging out with a famous soccer star ... but it seems there are still some harsh feelings over 2 years of them as a couple.