A pro MMA fighter nearly had his leg cut off his body in a gruesome car accident last year ... and now he's suing the California Department of Transportation, claiming it's responsible for his tragic injuries.

It's all in court docs obtained by TMZ Sports ... Chris Bonilla -- a former Muay Thai champ who also fought with the Glory kickboxing promotion -- says he crashed into an unmarked barrier on the side of a freeway last year.

Bonilla -- 31 years old at the time -- claims he was driving on the 10 Freeway in West Covina at around 6 AM when he needed to pull off to the side of the road for an unspecified reason.

But, Bonilla says when he did that ... he accidentally crashed into a K-rail that was NOT properly marked.

Bonilla says the results of the accident were devastating -- his car was split down the side ... severing his leg and breaking his clavicle in the process.

Chris says doctors were able to reattach his leg, but he's facing life-long issues and could have to have the leg eventually amputated.

Bonilla is now suing the California Department of Transportation for unspecified damages ... claiming it did not properly provide the necessary protections to prevent these of types of accidents from happening.