Ariana Grande's headlining set slayed at Coachella, and got an added boost by the long-awaited reunion of 4/5ths of her favorite boy band ... 'NSYNC.
Lance Bass, Joey Fatone, Chris Kirkpatrick and JC Chasez joined Ariana onstage Sunday night during her song "Break Up With Your Girlfriend, I'm Bored" -- which samples an old 'NSYNC hit -- then kicked the nostalgia into high gear by performing "Tearin' Up My Heart."
We broke the story of the surprise collab, including the fact Justin Timberlake wasn't going to be a part of it, but as you can see ... Grande filled in nicely for him.
At one point she announced to the crowd ... "I've been rehearsing my whole f**king life for this moment."
The big guest appearances during Ariana's set didn't stop with 'NSYNC -- Nicki Minaj also joined her and powered through some mic issues to do "Side To Side" and go out with a "Bang Bang."
Looks like JT missed out, but in his defense ... he just wrapped up his 115-show Man of the Woods tour the night before.
Maybe next year.