Tristan Thompson has hired a legal gun shooting down new cheating allegations from an Instagram model ... calling her a liar, and threatening to sue her if she keeps yapping.
The Celtics star -- who's back in with Khloe Kardashian now -- had his attorney, Marty Singer, fire off a cease and desist letter to Sydney Chase ... who recently claimed she had sex with Tristan since he and Khloe reunited. She also claims to have salacious texts from him.