An NHL rink looked more like the UFC octagon Thursday night ... 'cause a Red Wings player was straight-up KNOCKED OUT by one punch in an on-ice fight.
The violent scrap all went down during the Lightning's game against the Red Wings in Detroit ... when things between Tampa Bay's Barclay Goodrow and Detroit's Adam Erne got chippy and escalated quickly.
Late in the 2nd period ... Goodrow checked Erne into the boards -- and Erne didn't take kindly to the shove, throwing his stick at Goodrow's legs.
Goodrow returned the favor by attempting to trip Erne ... and then the guys went at it!!
They dropped the gloves, started swinging ... and a few seconds later, Goodrow landed a haymaker right on Erne's jaw.
You can see in the clip, Erne appeared to be knocked out on his feet ... and in an awesome gesture from Goodrow, the Lightning player held up his opponent by his jersey so his head wouldn't crash down on the ice.
Both players were assessed penalties for fighting ... and Erne had to head to the locker room to get checked out by trainers.
By the way, Erne may have lost the fight, but his team picked up the win in the game anyway ... notching a 6 to 4 victory.