That's how the Washington Mystics spent their first few minutes as WNBA champions ... 'cause after they beat the Connecticut Sun, 89-78 Thursday, they went back to their locker room for some champagne dives!!!

The vid is hilarious ... the team sprayed down their clubhouse with a ton of booze -- and with a protective layer of plastic laid on the ground, they decided to get down on the floor.

Shatori Walker-Kimbrough and Natasha Cloud counted backward from 3 ... dove across the carpet -- and good times were had by all.

Of course, the celebration didn't end there ... the team continued boozing throughout the night -- and were even spotted capping off the evening at local D.C. nightclub!!!

As for prior to the party ... the Mystics kicked ass on the hardwood -- and they had some special courtside visitors for the historic Game 5 -- Wizards stars John Wall and Bradley Beal!!

The two cheered their faces off all night long for the Mystics ... and were spotted giving congratulatory hugs to Mystics superstar Elena Delle Donne after the game.

By the way, Delle Donne was JACKED up for the win -- saying after the game, "My goodness, this feels so good!"