The former UFC middleweight titleholder and 1998 Pancrase Neo-Blood tournament champion was 37.
Tanner's agent John Hayner confirmed to the official UFC website that Tanner had gone on a camping trip by himself last Wednesday and was reported missing to authorities Friday.
"He was always planning on going on some sort of adventure," Hayner told UFC.com. "And he never needed the finer things or made a fuss about them. He just needed enough for gas, shelter, and training."
Officials at the Imperial County Sheriff Department suggests that Tanner died of excessive exposure to heat. The temperature in the desert over the weekend were upwards of 114.
Tanner first talked about a spiritual trip for "treasure" on Aug. 16 in one of his entries on his Spike TV blog.
"I'm hoping that very soon I'll be sitting out in the quiet of the desert beneath a deep blue midnight sky, listening to the calm desert breeze," Tanner said.
The idea of going into the desert was motivated by a friend's talk of treasure hunting and lost gold, and what he described as his "insatiable appetite for adventure and exploration."
Tanner said he would "imagine what might be found in the deep reaches of the untracked desert," and even described it as "an obsession of sorts."
Tanner considered himself a minimalist and wanted to carry as little equipment as possible on the trip.
"I plan on going so deep into the desert, that any failure of my equipment, could cost me my life."
The line was enough to draw concerns from fans. And on a post on Aug. 27, Tanner reassured readers that the trip would be safe.
"It seems that some MMA websites have reported on the story, posting up that I might die out in the desert, or that it might be my greatest opponent yet, etc. Come on guys. It's really common down in Southern California to go out to the off road recreation areas in the desert about an hour away from LA and San Diego.
"So my plan is to go out to the desert, do some camping, ride the motorcycle, and shoot some guns. Sounds like a lot of fun to me."
Tanner distanced himself from comparisions between him and the 2007 film "Into The Wild," which was based on a wanderer who gets rid of all his belongings to live alone in the wild, and ended up starving to death within four months after eating a poisonous plant.
"This isn't a version of 'Into The Wild.' I'm not going out into the desert with a pair of shorts and bowie knife, to try to live off the land. I'm going fully geared up, and I'm planning on having some fun."
Ironically enough, the song playing on Tanner's MySpace page is Eddie Vedder's "Guaranteed," a Golden Globe-winning song from the soundtrack.
UFC Middleweight Crown
Tanner's biggest accomplishment in MMA was capturing the UFC middleweight championship at UFC 51 in Feb. 2005. Tanner won by ground and pound strikes over the favored David Terrell, who came into the fight with a 24-second knockout victory over top ranked fighter and Olympic Silver Medalist Matt Lindland.
Tanner compiled an MMA record of 34-8 overall, and fought a total of 17 times for the UFC.