YouTube superstar Jake Paul WILLINGLY got in the ring with boxing phenom Ryan Garcia -- and let the dude BLAST AWAY on him ... and it's all on video!!!

The 22-year-old vlogger made the trip down to San Diego for some one-on-one gym time with the Flash ... and threw on some padding to do the #BodyShotChallenge.

Yeah, he's wearing padding ... but watch the video -- Ryan hits so damn hard, Jake gets rocked back around the ring and looks like he's in real pain.

At one point, it looks like Jake tries to get Ryan to stop, but the 19-0 fighter keeps throwing bombs!

"This is stupid," Jake said to the camera before taking the beating ... and we agree. V stupid.

Of course, Jake beat Deji in his first-ever fight last year in Manchester ... and has been BEGGING Eddie Hearn to let him take a pro fight like his big bro, Logan Paul.

Speaking of Logan, he also did the #BodyShotChallenge with Garcia a while back -- and it went about the same way!

Ryan 2, Paul Bros 0.