Sundlaugin (Icelandic pronunciation: [ˈsʏntˌlœyjɪn], the swimming pool) is a recording studio located near Álafoss in the town of Mosfellsbær in Iceland known for being the recording and rehearsal location of post-rock band Sigur Rós. The location was originally a swimming pool built in the 1930s which had been abandoned when Sigur Rós purchased it in 1999 and converted it and adjacent buildings into a studio.

The band originally intended to record their third album, titled ( ), in an abandoned NATO tracking base in the northernmost mountain in Iceland, but after inspection decided it was too impractical. Shortly after they found the abandoned pool lot in a rural neighborhood in Mosfellsbær. They bought the lot and transformed it into a studio. In order to fit the massive mixing console into the building, part of the roof was opened up and the console was lowered with a crane.

Much of the band's photography and artwork is taken from the surrounding landscape, such as the art found on the first album recorded in the studio, ( ).

The recording studio has also been used for recording, mixing and mastering (usually assisted by the studio's sound engineer Birgir Jón "Biggi" Birgisson) by a wide group of mainly Icelandic artists and bands, including:

• Agent Fresco
• The Album Leaf
• Alcest
• amiina
• Amusement Parks on Fire
• Björk
• Bubbi Morthens
• Emmanuel De La Paix
• Flying Hórses
• For a Minor Reflection
• Jakobínarína
• Julianna Barwick
• Langi Seli og Skuggarnir
• Mammút
• Mugison
• Múm
• Ólöf Arnalds
• Retro Stefson
• Seabear
• Self Defense Family
• Sin Fang
• Ske
• Slowblow
• Steindór Andersen
• Stórsveit Nix Noltes

As of 2020, Sundlaugin is owned by Sigur Rós keyboardist Kjartan Sveinsson.