Logan Paul laid the smack down on a dude ... knocking him unconscious with a vicious right-hand slap!!!

Ya gotta see, and hear, the video ... Logan winds up, cocks his right hand back with an open palm and connects with a brutal blow to the cheek that immediately sends his target tumbling to the ground!

After the one-slap knockout, Logan turns to the camera with a grin and FLEXES!!!

It all went down Tuesday during Logan's training session for an upcoming slap competition in Russia ... but the whole incident derailed his planned trip ... and now he's not going to compete.

Logan announced he's pulling out of the competition due to an "unforeseen and unfortunate" event that took place at practice. He says he no longer has the "desire to participate in any event that can result in serious injury to virtually anyone competing."

He further explained his decision on Mark Dohner's 'Livin' Large' Podcast.

Logan adds, "I feel my decision will preserve the health and wellness of everyone competing."