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, Hákon the Old, Magnus the Good, and Harald Hardrada. But they appear here expanded with additional material^[1] 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").

From internal evidence the book was being written in 1387 and was completed in 1394 or very soon after. The first page states that its owner is "Jonn Hakonar son" and that the book was scribed by two priests. One of them, "Jon prestr Þórðar son", scribed the contents from the tale of Eirík the Traveller down to the end of the two Olaf sagas and the other, "Magnús prestr Thorhallz sun", scribed the earlier and later material and also drew the illustrations.

Further material was inserted towards the end of the 15th century.

The manuscript first received special attention by the learned in 1651 when Bishop Brynjólfur Sveinsson of Skálholt, with the permission of King Frederick III of Denmark, requested all folk of Iceland who owned old manuscripts to turn them over to the Danish king, providing either the original or a copy, either as a gift or for a price. Jon Finnsson, who resided on Flatey ('Flat Island') in the fjord of Breiðafjörður on the northwest coast of Iceland, was then the owner of the book which was already known as the Flateyjarbók. At first Jon refused to release his precious heirloom, the biggest and best book in all of Iceland, and he continued to refuse even when Bishop Brynjólfur paid him a personal visit and offered him five hundreds of land. Jon only changed his mind and bestowed the book on the bishop just as the bishop was leaving the region.

The manuscript was given as a present from Bishop Brynjólfur to King Frederick III in 1656, and placed in the Royal Library of Copenhagen. In 1662, the bishop presented the king with a second medieval manuscript, the Codex Regius (Konungsbók eddukvæða). It and Flateyjarbók survived the Copenhagen Fire of 1728 and the Second Battle of Copenhagen in 1807 and were eventually repatriated to Iceland in 1971 as Icelandic national treasures. They are preserved and studied by the Árni Magnússon Institute for Icelandic Studies.

Flateyjarbók consists of the following texts:

Flateyjarbók is currently being translated into English by the Saga Heritage Foundation of Norway. The translator is Alison Finlay, professor of Medieval English and Icelandic Literature at Birkbeck, University of London. A Norwegian edition, translated by Edvard Eikill, is expected to be released in late 2019.