'Ágætis byrjun' is the second album by the Icelandic post-rock band Sigur Rós, which was first released in 1999. 'Ágætis byrjun' was recorded between the summer of 1998 to the spring of 1999 with producer Ken Thomas, and became Sigur Rós's breakthrough album, both critically and commercially.

'Ágætis byrjun' represented a substantial departure from the band's previous album Von, with that album's Cocteau Twins-esque dream pop and extended ambient soundscapes replaced by Jónsi Birgisson's now signature cello-bowed guitarwork and lush orchestration.

The album's title came from a friend hearing the first song they had written for the album, which would become the title track. After hearing the song, he said it was "a good beginning"; the name stuck. The name has also been translated as "An alright start."

Reissued on 2LP, and includes CD copy of the album as well.