Diddy got into a fight with a fellow inmate at his federal lockup in New Jersey -- and, after all the dust settled, he wound up in solitary confinement, TMZ has learned.

Multiple sources tell us Sean "Diddy" Combs went toe to toe with the other inmate this week inside FCI Fort Dix in NJ. We're told things got heated after the inmate dissed Diddy, who then decided to take matters into his own hands.

Our sources say the two inmates got into a scuffle and started pushing each other and throwing jabs. We're told Diddy "held his own" during the brawl, which was quickly squashed by prison staff who separated both men.

Our sources say Diddy was immediately tossed into the hole at Fort Dix. It's unclear if he's still in solitary as of this morning, but he could be facing more long-term consequences for the alleged fight.

Diddy's currently slated to get out of prison in February 2028 ... but that date assumes he earns credits for good behavior. If prison officials feel he was in the wrong in this cellblock showdown, he could be extending his stay in the federal pen.

TMZ contacted FCI Fort Dix and a rep tells us ... "We are not at liberty to disclose or provide internal information pertaining to any inmate in our custody. Such information would not be available for the public domain."

The Federal Bureau of Prisons also says it doesn't comment on "disciplinary matters, or other information regarding individual inmates due to privacy, safety, and security concerns."

We reached out to Diddy's reps ... so far, no word back.