Luka Doncic Celebrates Lakers Extension at Backstreet Boys Concert

What do you do when you ink a fat, new deal?? For Luka Doncic, there was only one answer -- hit up Las Vegas for the Backstreet Boys concert with Lakers execs!!

The two sides agreed to a massive three-year, $165 million extension Saturday to keep him in Purple and Gold through 2030 ... and before the ink was even dry, they made their way to Sin City to catch the legendary boy band at the Sphere.

Videos of the Lakers superstar attending the performance began to make rounds on social media ... and it's clear the vibes were at an all-time high after putting pen to paper.

Doncic and team president Rob Pelinka were seen jamming along to "I Want It That Way" ... along with everyone else in attendance.

They weren't the only big names in the crowd ... as Ashton Kutcher -- who was spotted chatting with Doncic -- and Mila Kunis were in the box right next to Luka and Co.

As if the day couldn't get better for the 26-year-old, he was able to head backstage to snap a flick with the group he called his favorite boy band back in April.

Jeanie Buss, future owner Mark Walter, Gabe Vincent, Rui Hachimura, Jarred Vanderbilt, Jaxson Hayes and Adou Thiero also tagged along for the celebratory trip to Sin City.

LeBron James was nowhere to be seen after Doncic signed his new deal ... but he reportedly FaceTimed with the Slovenian baller to congratulate him.