JLS (acronym for Jack the Lad Swing) are an English boy band consisting of members Aston Merrygold, Oritsé Williams, Marvin Humes, and JB Gill; Williams formed the band. They initially signed with Tracklacers production company New Track City and were the runners-up of the fifth series of the ITV reality talent show The X Factor in 2008, coming in second to Alexandra Burke. After their appearance on The X Factor, JLS signed to Epic Records. Their first two singles "Beat Again" and "Everybody in Love" were both number one on the UK Singles Chart.

The band's self-titled debut album was released on 9 November 2009 and has since sold over 1 million copies in the UK. JLS won the awards for British Breakthrough and British Single ("Beat Again") at the 2010 BRIT Awards. They won several awards at the MOBO Awards for Best song for "Beat Again" in 2009 and also Best Newcomer in the same year. In 2010, they won the MOBO Awards for Best UK act and Best Album. They also won their fifth MOBO in 2012 by winning Best Video for "Do You Feel What I Feel?". They won the title of the UK's hardest-working band for two consecutive years, in 2011 and 2012.

In 2010, JLS signed a record deal with the US record label Jive Records and released "Everybody in Love" as their debut and only US single, but it failed to chart. "The Club Is Alive", the lead single from their second studio album, was released in the UK in July 2010 and earned the band their third number-one on the UK Singles Chart. Their single "Love You More" was the official single for Children in Need in 2010 and gave the group their fourth number-one single in the UK. Their single "She Makes Me Wanna" featuring Dev was their fifth UK number one. As of 2012, their debut album and single have been named one of The X Factor's top ten biggest-selling debut singles and albums. As of 2013, they were the 16th-richest reality TV stars in the UK, with an estimated fortune of £6 million per member, thus giving the band a financial worth of approximately £24 million.

In May 2013, after seven years together, JLS announced that they would go their separate ways after releasing a greatest hits album and one last UK and Ireland arena tour. In February 2020, the band announced a reunion, with the Beat Again Tour due to start in November 2020, but it was postponed to June 2021 due to the ongoing COVID-19 pandemic and then postponed to October 2021.

According to the British Phonographic Industry (BPI), JLS has sold 2.3 million albums and 3.6 million singles in the UK.