MMA rising star Maycee Barber was knocked out cold by her opponent, Alexa Grasso, during Saturday's UFC Fight Night 271 -- and video of the takedown is downright terrifying.  

Check it out ... in the first round, Barber and Grasso are going toe to toe, kicking and throwing jabs until Grasso connects with a hard left to the face.

Barber falls on her ass as Grasso goes in for the kill, putting Barber in a chokehold before she goes unconscious with her eyes staring into space.

The referee quickly ended the fight and felt for a pulse on Barber's neck while a medical professional even bolted inside the Octagon to provide first aid.

After some scary moments, Barber was able to get to her feet and stand with Grasso, who was announced the winner in the middle of the Octagon.

Close call for Barber.