Jimmy Fallon thanks Jonas Brothers for inviting him on The Tour

Jimmy Fallon delighted Jonas Brothers' fans at their concert on Sunday night.

The late-night host made an unexpected cameo at Yankee Stadium in the Bronx, New York. He led the audience in a sing-along of The Killers' "Mr. Brightside."

“Huge thanks to @jonasbrothers for giving me the surprise guest spot on The Tour tour. And thank you to @yankeestadium for turning it into the world’s biggest karaoke party. This is one of those nights I will never forget,” Fallon posted on social media along with a video.

“I brought my boom box,” he also yelled to the crowd as he showed off said boom box, which was attached to a cross-body strap.

“Are we having fun yet?” he added, to cheers from the crowd.

At Sunday night's event, Fallon appeared after the intermission to reintroduce the band. The show was the second stop on the Jonas Brothers' The Tour tour, which began Saturday night at Yankee Stadium.

On their current tour, the Jonas Brothers — Joe, Nick, and Kevin — are performing songs from five distinct albums, including this year's The Album.

“It’s our most ambitious show we’ve ever put on, in the sense that building out five albums in one night was a challenge that I don’t think we fully understood after we’d already put it on sale,” Nick told People of their current tour. “[But] it’s amazing to just go back and look at the road that brought us to this moment now.”

The Tour includes 90 performance dates and will head overseas after the U.S. portion. Next up is Boston on Tuesday.