Mike Posner's epic trek across the country is derailed thanks to a rattlesnake bite so bad he had to be airlifted to a hospital to save his life.

The singer-songwriter was making his way across Colorado Wednesday and had already racked up 16 miles on the day with a goal of 8 more when he got bit by a baby rattler. We're told the snake jumped out of nowhere on his path and sunk its fangs into his left leg.

According to Mike, "That venom is no joke!"

This pic of his feet shows where the venom caused swelling from the bite on the ankle -- pretty gnarly.

Fortunately, he says he got to the hospital and received the anti-venom in time, courtesy of an emergency helicopter ride and the expert staff -- but he'll be laid up for a few days and won't be able to continue his walk for several weeks.

On the bright side, Posner says he's enjoying the hospital's AC and a nice bed ... which he hasn't had in quite a long time. He vows to get back on "The Walk" as soon as he's able, though.

We had Mike on "TMZ Live" in April and he broke down the purpose of his journey, which was inspired by the deaths of his father and Avicii. He started on April 15 in New Jersey, and hoped to make it to Venice, CA by October.

He's releasing new music along the way whenever he crosses a state line ... most recently, “Prince Akeem” ft. Wiz Khalifa as he got to Colorado.

The bite is definitely a setback, but Mike insists a lil ol' snake isn't gonna stop him.