The Boss is back.

Bruce Springsteen surprised the audience at the Stand Up for Heroes fundraiser for the Bob Woodruff Foundation at Lincoln Center's David Geffen Hall in New York on Monday night.

Springsteen joined John Mellencamp for the song “Wasted Days” from Mellencamp's 2022 album, “Strictly a One-Eyed Jack,” then stuck around to perform four solo acoustic songs. He also told ribald double-entendre jokes, as he's done in previous appearances for the event, which raises funds for veterans causes.

Springsteen was not billed as a performer this year as he was scheduled to be on road with the E Street Band. Since the Stand Up lineup was announced, Springsteen postponed the E Street Band shows for the rest of the year due to his bout peptic ulcer disease.

Springsteen performs 'Power of Prayer', 'Dancing in the Dark'

Dressed in black, the Boss was in fine form and superior voice, showing no ill effects of the ulcers. Springsteen performed “Addicted to Love,” a gentle ballad from the recent romantic-comedy “She Came to Me.” It was the song's live debut.

Springsteen exhibited a sweet vibrato on “Power of Prayer” from the 2020 E Street Band album “Letter to You,” and he rocked his Takamine acoustic guitar for “Working on the Highway.” The Takamine must have some pretty thickly-gauged strings judging by the workout Springsteen was giving it.

“If at first you don't succeed, don't try to sky dive,” quipped Springsteen, 74, after “Working on the Highway,” the only joke we can safely publish here. (Stand Up For Heroes is part of the New York Comedy Festival.)

Then came “Dancing in the Dark,” where Springsteen sang part of the song off mic, his voice filling the theater without vocal amplification. The Boss responded with a smile when he was "Bruuuuced” during the song.

Springsteen, who sang backup for the house band on a song at the Oct. 29 New Jersey Hall of Fame ceremony in Newark, was introduced by Mellencamp after the Stand Up for Heroes show was stopped due to a medical emergency in the audience. Once the issue was addressed, comic Jon Stewart reintroduced Mellencamp, who then introduced Springsteen to the surprise of the audience.

“I hope everybody's all right. I didn't want to make any people sick,” Mellencamp said. “I'm going to bring out one of the best songwriters of our generation, and he's my big brother and I've looked up to him my whole life. Ladies and gentlemen, Bruce Springsteen.”

Mellencamp had performed a solo acoustic “Jack and Diane” and “Small Town,” accompanied by an electric guitarist, violinist and accordion player before the show was temporarily paused.

'We're heartbroken':Bruce Springsteen postpones shows with E Street Band to treat peptic ulcer disease

Josh Groban, Rita Wilson, Tracy Morgan, Jon Stewart also took to the stage

Earlier in the show, married duo Michael Trotter Jr. and Tanya Trotter, aka the War and Treaty, delivered a moving and artistically heightened set of three songs, which included a sublimely soulful version of “God Bless America.”

Michael is a veteran who fought in Iraq, and he told how he learned to play piano there on an instrument once owned by Saddam Hussein. He also told of how a pre-teen translator he befriended in Iraq was killed by an explosion before he turned 13.

The experience caused Michael to consider suicide after he returned home until Tanya asked him for “Five More Minutes.”

Josh Groban and Rita Wilson also performed sets of music, and comedians Tracy Morgan, Jimmy Carr, Ronny Chieng, Shane Gillis and Stewart performed stand-ups. First Lady Jill Biden and Prince Harry appeared in taped addresses.

On stage, Craigslist founder Craig Newmark donated $10 million to support the Woodruff Foundation’s initiative to improve mental health. The Woodruff Foundation forges partnerships and unites leaders in government, the military, business and philanthropy in support of veterans.

Bob Woodruff is the ABC correspondent who was wounded in Iraq in 2006. At 8 p.m. Friday, Nov. 10, he's featured in a new ABC special, “After the Blast: The Will to Survive,” which shows Woodruff and his team's journey back to Iraq.