Adrian Peterson's transition from the football field to the boxing ring is clearly going just fine -- the former Vikings star showed this week he has a NASTY right hand, flattening a guy with it during a recent sparring sesh.
The 33-year-old posted video of the impressive KO on Monday ... showing him crushing his opponent with a right that caught the man square in the jaw.
In the clip, you can see the dude hit the canvas almost as soon as the punch landed -- and he was so dazed from the blow, he couldn't immediately take Peterson's offer for help up.