Fóstbrœðra saga () or The Saga of the Sworn Brothers is one of the Icelanders' sagas. It relates the deeds of the sworn brothers Þorgeirr and Þormóðr in early 11th century Iceland and abroad.
Þorgeirr is a capable and insanely brave warrior. He kills people for trifles and for sport.
Þormóðr is a more complicated character; warrior, trouble-maker, womanizer and poet. The saga contains poetry attributed to him, including parts of a lay on his blood brother.
The saga is preserved in several different versions and a long-standing controversy centered on which manuscripts represented the most original version. In particular the debate focused on several unusual "clauses" in the saga which do not fit in with the conventional saga style.
The composition of Fóstbrœðra saga is probably to be dated to ca. 1200 (according to Sigurður Nordal in Björn K. Þorólfsson and Guðni Jónsson 1943: lxxii).
The skaldic stanzas attributed to Þormóðr kolbrúnarskáld Bersason appear genuine (according to Guðni Jónsson in Björn K. Þorólfsson and Guðni Jónsson 1943: lxi); he would have composed ca. 1010-1030 (according to Guðni Jónsson in Björn K. Þorólfsson and Guðni Jónsson 1943: lxix).
Björn K. Þórólfsson and Guðni Jónsson (Eds.) (1943). Íslenzk fornrit VI - Vestfirðinga sögur. Reykjavík: Hið íslenzka fornritafélag. No ISBN.
Jónas Kristjánsson (1972). Um Fóstbræðrasögu. Reykjavík: Stofnun Árna Magnússonar. No ISBN.