The former "Vanderpump Rules" star spilled the tea with his wife Brittany Cartwright on Wednesday's "Watch What Happens Live" -- telling Andy Cohen he wasn't shocked at all when the Scandoval drama came to light. He also claims he was practically a witness for the infamous Miami incident years ago.

Fans of the show remember the long-standing rumor that Tom cheated on Ariana with a girl in Florida at the very beginning of their relationship ... and Jax says he was in the next room when it went down.

He's certain this has been a frequent thing with Tom, but still won't go into specifics on what he knows.

Jax also talks to Tom's business partner Tom Schwartz on the daily, and claims the guy knew about all of this, too -- swearing on his own child and even quoting what Schwartz told him.

Brittany and Jax go on to connect some dots from the past that seemed harmless at the moment ... stuff like going to the same parties together. Jax even recalls a time when Ariana told Tom her grandma wasn't doing well while he was at a party with Raquel, but he chose to stay instead of leave and comfort his GF.

For what it's worth, this is all coming from Jax -- someone who's known as the OG cheater on the Bravo series, and he seems to recognize that irony, too.