Jón Þór "Jónsi" Birgisson (pronounced:  ( listen)) (born 23 April 1975) is the guitarist and vocalist for the Icelandic post-rock band Sigur Rós. He is known for his use of a cello bow on guitar and his falsetto or countertenor voice. He is blind in his right eye from birth as a result of a broken optic nerve from the brain. He is openly gay. Apart from Sigur Rós, Jónsi also performs together with his partner Alex Somers as an art collaboration called Jónsi & Alex. They released their self-titled first book in November 2006, which was an embossed hardcover limited to 1000 copies, along with their first album, Riceboy Sleeps, in July 2009. On 1 December 2009, Jónsi's official website, jonsi.com, was launched in anticipation of his debut solo album, Go, which was released the week of 5 April 2010. After the release of the album, Jónsi promptly started a worldwide tour across North America and Europe, featuring songs from the album plus a few other selections, planning to tour from March to September.

In late January 2010, Jónsi announced that Sigur Rós was on "an indefinite hiatus", as the band had scrapped plans for a new album previously announced to be released in 2010, saying that "they were just rumours". The band decided to take the year off, as several members of Sigur Rós recently had children, and as Jónsi developed his solo career. The band is now back together again and released the album Valtari on 28 May 2012 with a following tour starting in August of the same year, and more recently the album Kveikur on 18 June 2013.