DeMarcus Cousins didn't let a torn ACL hold him back on the biggest day of his life ... so says Matt Barnes, who tells TMZ Sports the NBA superstar was dancing and in great spirits at his wedding over the weekend!!!

The 29-year-old Lakers center married his longtime girlfriend, Morgan Lang, in Atlanta on Saturday -- just 2 weeks after seriously injuring his left knee in a Vegas training sesh.

The event looked like NBA All-Star Weekend with huge stars like Barnes, Anthony Davis, Draymond Green, Andre Iguodala and more in attendance.

As for the dancing, Boogie didn't do anything stupid -- he was cautious -- and left the wild dance moves to his new wife!!!

You can see in the footage ... Cousins was moving nice and slow during his first dance with his wife. There will be plenty of time for Boogie to boogie after he gets healthy.

We talked to Barnes about DC's morale during his big day ... and while Matt says he's sure the guy is frustrated about the recent bad luck, Cousins didn't show it on Saturday.

"He's been through a lot of late, so I'm happy to see him smile, be able to take his mind off his injury and enjoy his day," Barnes told us at LAX.

BONUS -- Matt Barnes had a BIG problem on the day after the wedding ... he had a bunch of weed but didn't want to take it to the airport.

What ensued is one of the funniest scavenger hunts we've ever seen -- and Barnes posted a road map to his stash on social media!!!

Good luck!!!