The "NIGHTS LIKE THIS" singer turned 21 Saturday, so naturally he was in Las Vegas ... ordering a round of shots 30 minutes into his big day.

LAROI hit up XS Nightclub inside Wynn Las Vegas around 12:30 AM with a 10-person entourage ... his crew sucked down tequila shots and he sipped on his drink of choice -- an Aperol Spritz. Those first legal beverages hit different!!!

The Kid was definitely feeling the love, because the entire club serenaded him with the "Happy Birthday" song ... and his face was plastered all over the fake thousand-dollar-bill confetti raining down from above.

LAROI even had his baby photos lit up on the LED screens inside the club ... plus a special b-day message.

There was cake too ... a three-tiered dessert decorated with LAROI's album covers and a massive, silver "21" on top.

LAROI's birthday bash morphed into a mini concert ... when he joined DJ Gryffin inside the booth to grab the mic and perform a couple of his hit songs ... "Stay" and "Girls." The Kid also played a new, unreleased song named for his favorite drink ... "Aperol Spritz."

Sounds like The Kid had a helluva time ... before jumping off stage, he told the crowd how excited he was to finally be 21 years old in Las Vegas ... and he didn't leave the club until 3 AM.