Icelandic code of laws, instituted by the Althing in 1281, and named for its main co-author, the lawspeaker Jón Einarsson. The Jónsbók is based on the 1274 land-law of Magnus VI Lagabøter, King of Norway.

With only minor changes, the Jónsbók was the prevailing law in Iceland until the introduction of absolutism (as part of the absolutist monarchy of Denmark-Norway) in 1662 - and it has never been completely revoked or superseded.

The first printed edition of the Jónsbók was published in 1578, but many manuscript editions exist.