Alan Ritchson's not voting for his childhood classmate Matt Gaetz anytime soon ... 'cause he used some choice verbiage to take a shot at the dude who he calls his adversary.

The "Reacher" star sat down for an interview with GQ -- released Wednesday -- when he was asked about the former congressman who he met in Niceville, Florida where his family settled when he was a kid.

It sounds like the two weren't having sleepovers and going on double dates ... 'cause Ritchson says, "That mother******. We are adversaries. It's shocking to me that the panhandle of Florida continues to vote for somebody—knowing everything we know about him and the promises that he's made behind closed doors about pardoning certain criminals—he's just not a good dude!"

AR says he considers getting into politics to try and beat someone like Gaetz ... but, he adds he doesn't think he's "duplicitous enough to succeed" in the field.

Ritchson goes on to discuss more topics -- from drug usage and a shocking suicide attempt to his television career and fatherhood -- but, it's clear the topic of Gaetz is really the thing that gets him the most worked up.

As you know ... Gaetz was a Congressman from 2017 until he resigned last year. President Donald Trump initially announced him as his pick for Attorney General.

A House Ethics Committee report accused Gaetz of engaging in sexual activities with a 17-year old -- among other troublesome allegations -- and Gaetz withdrew his name from consideration.

Sounds like Gaetz may want to steer clear of Ritchson ... or, he may go full Jack Reacher on him!