Nirvana tore up the stage Thursday night at L.A.'s FireAid benefit show ... giving the lucky crowd an unexpected and all-around epic reunion with some special guests.

The rock band was not announced ahead of time as part of the star-studded lineup ... making their reunion that much more special.

The group's three surviving members -- Krist Novoselic, Dave Grohl and Pat Smear -- took the stage at the Intuit Dome near the end of the show and welcomed a rotating list of singers to fill in for the late Kurt Cobain. St. Vincent contributed vocals for "Breed," Kim Gordon helped the band perform "School" and Joan Jett filled in for "Territorial Pissings."

Things got even sweeter when Dave's 18-year-old daughter Violet Grohl took over the lead mic for a rendition of "All Apologies." Just look at her ... she seems confident and comfortable on stage, just like her famous father.

Dave shares Violet plus two other girls -- Harper and Ophelia -- with his wife, Jordyn Blum. He infamously welcomed another baby girl outside their marriage in September, but said he was focused on saving his relationship with his wife and older kids.

Nirvana disintegrated after KC's death in 1994. They reunited in 2012 to record "Cut Me Some Slack" with Paul McCartney, and again two years later for their induction into the Rock and Roll Hall of Fame. They last performed as a group in 2018 at San Bernardino's Cal Jam.