Jón Þór "Jónsi" Birgisson is an Icelandic musician; he is the vocalist and multi-instrumentalist for the Icelandic post-rock band Sigur Rós. He is known for his use of a cello bow on guitar and his "angelic" falsetto or countertenor voice.

Sigur Rós is an Icelandic post-rock band that formed in 1994 in Reykjavík. It comprises lead vocalist and guitarist Jón Þór "Jónsi" Birgisson, bassist Georg Hólm and keyboardist Kjartan Sveinsson. Known for their ethereal sound, frontman Jónsi's falsetto vocals, and their use of bowed guitar, Sigur Rós incorporate classical and minimal aesthetic elements. Jónsi's vocals are sung in Icelandic and non-linguistic vocalisations the band terms Vonlenska. They have released eight studio albums, and attracted critical and commercial attention with their second album Ágætis byrjun.