Gissur Thorvaldsson (1208 – 12 January 1268; Modern Icelandic: Gissur Þorvaldsson [ˈcɪsːˌʏːr ˈθɔrˌval(t)sˌsɔːn]; Old Norse: Gizurr Þorvaldsson [ˈɡit͡sˌurː ˈθorˌwɑldsˌson]) was a medieval Icelandic chieftain or goði of the Haukdælir family clan, and great-grandson of Jón Loftsson.

Gissur played a major role in the period of civil war which is now known as Age of the Sturlungs: he fought alongside Kolbeinn the Young against the forces of Sturla Sighvatsson of the Sturlungar clan in the Battle of Örlygsstaðir in 1238 and led the force of men who murdered saga-writer Snorri Sturluson in 1241, at the behest of Haakon IV, King of Norway,^[1] who demanded Snorri's return to Norway for disobeying his order to stay at his court following Jarl Skúli's failed coup. In 1253, Gissur's son was wedded to Sturla Þórðarson's daughter Ingibjörg, as a part of an attempt at ending the conflict between the Haukdælir and Sturlungar. Shortly after the wedding, Eyjólfr ofsi, another member of the extended Sturlungar, attacked Gissur's household at Flugumýri, in what is termed the Flugumýrarbrenna (Flugumýri Arson), a devastating attack that destroyed his paternal family line (Gissur survived the attack by hiding in a barrel of skyr.^[2]) The event was clearly traumatic for Gissur, and caused him to campaign against the burners, and compose poetry.^[3] In 1258, he was made Earl of Iceland (Icelandic: jarl) for his loyal service to the king. He held this title until his death.

Gissur worked actively to promote the Old Covenant (Icelandic: Gamli sáttmáli), an agreement which brought Iceland under the sovereignty of the Norwegian crown in 1264. The covenant is hence sometimes known as Gissur's Covenant, or Gissurarsáttmáli.