Tristan Thompson has his legal guns putting the heat back on the Instagram model claiming she has salacious texts from him … telling Sydney Chase to put up or shut up.

The Celtics star — who’s with Khloe Kardashian — had his attorney, Marty Singer, fire off a legal threat to Chase … who claims she had sex with Tristan after he got back with Khloe.

In the letter, obtained by TMZ, Singer says Chase is refusing to prove the existence of the alleged texts and it can only lead to one “inescapable conclusion … that they do not exist.”

As we reported … Tristan’s legal team previously fired off a cease and desist to Chase, claiming they never had a relationship and he never sent her any texts … and she initially denied receiving the threat.

Chase has since changed her tune, acknowledging she received the cease and desist after it was sent to another email address … and now Tristan’s legal team says she keeps “changing the purported ‘facts’ to fit [her] story.”

Tristan’s attorney says Chase is making false claims about Thompson to get her 15 minutes of fame … and TT’s legal team is demanding she stop defaming him, saying she’s risking a lawsuit if she keeps yapping.