Jason 'Mayhem' Miller Arrested -- Ex-UFC Fighter Busted in Church ... BUCK NAKED

3:37 PM PT -- Naked is in ... TMZ just obtained Jason Miller's mug shot after his crazy naked arrest today in an Orange County church ... and he's wearing the paper suit reserved for naked arrestees. Just like Randy Travis.

Recently-axed UFC fighter Jason "Mayhem" Miller was arrested today in Orange County, TMZ has learned -- after allegedly breaking into a church, spraying a fire extinguisher all over the place, and getting buck naked.

Law enforcement sources tell TMZ, the O.C. Sheriff's Office received a call early this morning from the Mission Hills Church in Mission Viejo about a possible burglary in progress.

When the deputies arrived, we're told they found the first and second floors covered in white fire extinguisher spray. Cops say the place was also trashed -- scattered CDs, books, and broken pictures.

When officers reached the second floor, we're told they found Miller naked on some couch -- totally awake and coherent.

Miller -- who also hosted the MTV reality show "Bully Beatdown" -- was taken into custody for burglary, and is currently being medically evaluated. It's unclear if Miller was under the influence.

It's worth noting -- UFC honcho Dana White cut Miller earlier this year, claiming "some crazy sh*t happened backstage" at UFC 146 and Miller was fired.