Jack Michael Antonoff (born March 31, 1984) is an American singer, songwriter, and record producer. He is the lead vocalist of the rock band Bleachers, and previously the guitarist and drummer for the pop rock band Fun and the lead vocalist of the indie rock band Steel Train. Antonoff has produced and co-written songs with other music acts such as Taylor Swift, Lorde, Lana Del Rey, St. Vincent, Pink, Kendrick Lamar, Sabrina Carpenter and Doja Cat.

Antonoff has won eleven Grammy Awards. As part of Fun, he was awarded the Best New Artist and the Song of the Year for "We Are Young" (2011). He gained prominence as a music producer following his works with Swift, leading to three Album of the Year wins from her albums 1989 (2014), Folklore (2020), and Midnights (2022). His other Album of the Year nominations include Lorde's Melodrama (2017), Swift's Evermore (2020) and The Tortured Poets Department (2024); Del Rey's Norman Fucking Rockwell! (2019) and Did You Know That There's a Tunnel Under Ocean Blvd (2023); and Carpenter's Short n' Sweet (2024).

Having won Producer of the Year three consecutive times from 2022 to 2024, Antonoff has been credited by critics with having influenced the popular music trends of the 2010s and 2020s decades. Songs he contributed to—from "We Are Young" to Swift's "Look What You Made Me Do" (2017), "Cruel Summer" (2019), "All Too Well (10 Minute Version)" (2021), "Anti-Hero" (2022), "Is It Over Now?" (2023), and "Fortnight" (2024), Sabrina Carpenter's "Please Please Please" (2024) and "Manchild" (2025), and Kendrick Lamar's "Squabble Up" and "Luther" (both 2024)—have topped the Billboard Hot 100 chart.

He has curated film soundtracks as well, including One Chance (2013), Fifty Shades Darker (2017), Love, Simon (2018), and Minions: The Rise of Gru (2022); singles from the first two soundtracks, "Sweeter Than Fiction" by Swift and "I Don't Wanna Live Forever" by Swift and Zayn Malik, have garnered nominations for the Grammy Award for Best Song Written for Visual Media.

Antonoff was born on March 31, 1984, in Bergenfield, New Jersey. He is the second of three children of Shira (Wall) and Rick Antonoff. He is the younger brother of fashion designer Rachel Antonoff. His younger sister, Sarah, died of brain cancer at the age of 13 when Jack was a senior in high school. The event had a profound effect on him, "my whole career has been revisiting that through a different lens." Antonoff is Jewish. He grew up in New Milford, New Jersey, and Woodcliff Lake, New Jersey and attended elementary school at the Solomon Schechter Day School of Bergen County. He and Rachel commuted to Manhattan to attend the Professional Children's School.

During his sophomore year of high school, in November 1998, he and several friends from elementary school formed a punk rock band called Outline. They released a self-titled EP through Lifetime Records, a friend's record label, in January 2000, which was recorded by Jack's childhood friend, Heath Miller of Excess dB Entertainment. Antonoff initially sang in the group until early 2000 when Eddie Wright took over on vocals. With the addition of Wright, they recorded six songs which were later released as 6 Song Demo in the summer. They released an album, A Boy Can Dream, in July 2001 through Triple Crown Records. When they were 15, Antonoff and his Outline bandmate used a DIY guide to book shows in numerous states, including Florida and Texas, and borrowed Antonoff's parents' minivan to travel in. During the tour, Outline played in venues such as anarchist bookstores, while the oldest member of the band drove because he was 18 years old. Antonoff explained in 2014: "Half the time no one would show up or the equipment would be too fucked up to play... but that's when I fell in love with touring." The band lasted until 2002, when Steel Train would sign to Drive-Thru Records.

In summer of 2001, Antonoff and friend Scott Irby-Ranniar formed the band Steel Train, playing their first ever show at Club Krome in South Amboy, NJ on September 15, along with future labelmates Allister and The Early November, who were about to sign to Drive-Thru Records. Antonoff was the lead singer, and they recruited drummer Matthias Gruber. The band then convinced two of their friends from the band Random Task (at the time managed by Heath Miller, promoter of Club Krome at Excess dB Entertainment), Evan Winiker and Matthew Goldman, to drop out of college to join the new band. Steel Train secured a recording deal with Drive-Thru Records. The group was popular on the jamband festival circuit and Antonoff has said he has applied that grassroots mentality to his future projects.

In 2008, Nate Ruess (formerly the frontman of the Format) asked Antonoff to join him and Andrew Dost (formerly of Anathallo) in a new band, which became Fun. Antonoff was already well acquainted with Ruess and Dost, as their former bands had all toured together, meeting at a show at Club Krome on June 11, 2003. The new band released its debut album, Aim and Ignite, in 2009. Fun's second album, Some Nights (2012), produced the band's first number-one hit single, "We Are Young". The song was co-written by Antonoff with Ruess, Dost, and Jeff Bhasker. Fun. then played with their musical heroes Queen in September 2013 at the iHeartRadio Music Festival, which was held at the MGM Grand Garden Arena in Las Vegas. Antonoff played Brian May's guitar during the rehearsal, which he described as the "most surreal experience ever." The band then released a free six-song EP in December 2013, titled Before Shane Went to Bangkok: Fun. Live in the USA.