The rapper -- who's currently incarcerated on federal racketeering charges -- is expected to testify next month about the kidnapping and assault he claims he fell victim to in 2018. Cops arrested a suspect in the case, who pled not guilty, and also claims Tekashi made up the whole thing.

FYI, the guy calling BS -- Anthony Ellison -- was formerly part of Tekashi's management team and cops believe he's affiliated with the Nine Trey Bloods, the same gang Tekashi was with prior to his bust. Bottom line ... they know each other well.

That's what makes this situation messy for Tekashi -- he's currently preparing to point the finger at this guy, and the potential ramifications are very serious. Sources connected to the case tell TMZ, 6ix9ine's crapping his pants over testifying -- worried about how his mother and brother could be harmed, and also for his own safety behind bars. We're told he knows this could make him a marked man for life.

Case in point ... look what happened to notorious mobster Whitey Bulger last year in prison.

We already know 6ix9ine has shelled out thousands for security to protect his mom, but even with that ... we're told he feels there's nowhere for his loved ones to hide if the Nine Trey Bloods want to get even with him for snitching.

There's really no way out of this jam for Tekashi ... he's required to cooperate with the feds, so if they want him to testify, he's gotta sing. His last hope would be for Ellison and prosecutors to strike an 11th-hour deal.

Barring that ... we're told Tekashi will look kinda different on the stand -- same face tats, but hardly any rainbow hair these days. Not a ton of salon time in prison ...