Josh Duggar is on the move within the federal prison system, getting transferred to a penitentiary that caters to inmates who need medical and mental health services ... TMZ has learned.

The "19 Kids and Counting" star turned convicted sex offender was transferred May 29 from a minimum-security facility, FCI Seagoville in Texas, to the Federal Medical Center in Forth Worth ... according to the Federal Bureau of Prisons.

Duggar had been locked up at FCI Seagoville from June 2022 to May 2026 ... but the BOP refused to explain the reason behind the move, citing security concerns.

That said, on the BOP website, FMC is described as an administrative security federal medical center with a detention facility. The BOP notes that all federal inmates receive essential medical, dental, and mental health services.

And get this ... Duggar might soon be rubbing elbows with the "Tiger King," Joe Exotic, who is also locked up at FMC Fort Worth. Joe, as you know, has been fighting prostate cancer for years in prison after his 2019 conviction in a failed murder-for-hire plot to kill a rival animal welfare activist, Carole Baskin. He is currently serving a 21-year sentence.

Duggar, on the other hand, was sentenced to 12.5 years behind bars after being convicted in December 2021 for receiving and possessing child pornography.