Flateyjarbók is an important medieval Icelandic manuscript. It is also known as GkS 1005 fol. and by the Latin name Codex Flateyensis.
Flateyjarbók is the largest medieval Icelandic manuscript, comprising 225 written and illustrated vellum leaves. It contains mostly sagas of the Norse kings as found in the Heimskringla, specifically the sagas about Olaf Tryggvason, St. Olaf, Sverre, Hakon the Old, Magnus the Good, and Harald Hardrada. But they appear here expanded with additional material not found elsewhere (some of it being very old) along with other unique differences. Most—but not all—of the additional material is placed within the royal sagas, sometimes interlaced. Additionally, the manuscript contains the only copy of the eddic poem Hyndluljóð, a unique set of annals from creation to 1394, and many short tales not otherwise preserved such as Nornagests þáttr ("the Story of Norna Gest").
Especially important is the Grœnlendinga saga ("History of the Greenlanders"), giving an account of the Vinland colony with some differences from the account contained in Eiríks saga rauða ("History of Eirík the Red"). Here also are preserved the only Icelandic versions of the Orkneyinga saga ("History of the Orkney Islanders") and Færeyinga saga ("History of the Faroe Islanders").
Flateyjarbók contains a short story called Völsa þáttr, which offers an account of a pagan ritual in which two men lift the lady of the household over a door frame to help her look into the otherworld; this ritual is also recorded by a 10th-century Arab Muslim writer named Ahmad ibn Fadlan, who witnessed this ritual firsthand during a Norse funeral.