The 59-year-old went into detail about the encounter in an IG video ... saying he was running in Marin Headlands when a coyote ambushed him.

"I just had something rather terrifying happen," Karnazes said. "I'm out on a 150-mile trail run and I got attacked by a coyote."

"That's a first. It knocked me over and thankfully I'm running with poles so I whacked it and it ran away."

But, Karnazes did suffer injuries ... you can see in his selfie video he had blood all over his mouth, chin and shirt.

"Not sure what I’m gonna do," he said in the clip. "I guess I gotta keep going or else it'll probably come back for me."

After the experience, Karnazes -- who has won the Badwater Ultramarathon and Vermont Trail 10 Mile Endurance Run -- advised people to stop feeding coyotes on the trail, with the hopes it'll keep the wild animals further away from runners.

"If you see someone feeding a coyote, please say something," Karnazes said.

"The local Rangers are doing the best they can, but we trail runners are out in these areas more than anyone. Thank you. It's best for everyone, coyotes included. #letwildlifebewild"

FYI, this ain't the first time Karnazes has been attacked by a wild animal -- he also said on his social media page that he's had a scary run-in with a shark in the past as well.

"I've been attacked by a shark, and now a coyote," he said. "Both incidents were terrifying."

Karnazes has competed in several running and swimming events throughout his career -- in fact, he swam across the San Francisco Bay and ran a marathon in every state in the U.S. for 50 consecutive days in 2006.

He's also a columnist for Men's Health and the author of "Ultramarathon Man: Confessions of an All-Night Runner."