Britney Spears and ‘N Sync will topline Nickelodeon’s “Big Holiday Help-a-thon,” a bi-coastal, two-hour primetime special intended to champion volunteerism.

The 6th annual event is part of the kids’ cable network’s $30 million global commitment to “The Big Help” program.

Hosted by Nick’s Kenan Thompson, the “Help-a-thon” will cover both coasts, with “The Big Help Holiday Jam” — featuring Spears, ‘N Sync and other performers — on Nov. 22 in L.A. and a live dance party on Dec. 11 in New York. Mike O’Malley will host the New York City portion of the event, which will feature the taped concert.

“It will look like a giant dance party with a holiday theme — like Times Square meets Tavern on the Green at Christmastime,” said Albie Hecht, Nick’s president, film and TV entertainment.

Big Help spokesperson Mariah Carey, as well as Elton John and Mia Hamm, have signed on as “Team Big Help All-Stars,” and will appear on the show in a series of taped behind-the-scenes vignettes about their own personal causes.