The Alþingi (anglicised as Althing or Althingi) is the national parliament (literally: "[the] all-thing", or general assembly) of Iceland. It is the oldest extant parliamentary institution in the world together with the Jamtamot, today Jämtlands läns landsting, of Jämtland County, Sweden. The Althing was founded in 930 at Þingvellir, the "assembly fields" or "Parliament Plains", situated approximately 45 km east of what later became the country's capital, Reykjavík. This event marked the beginning of the Icelandic Commonwealth. Even after Iceland's union with Norway in 1262, the Althing still held its sessions at Þingvellir until 1799, when it was discontinued for 45 years. It was restored in 1844 and moved to Reykjavík, where it has resided ever since. The present parliament building, the Alþingishús, was built in 1881, of hewn Icelandic stone.

Das Althing, isl. Alþingi, ist das Parlament Islands. Es folgt seit 1991 dem Einkammersystem. Das Althing geht bis ins Jahr 930 zurück und ist damit das älteste bestehende Parlament der Welt. Als die älteste durchgehend existierende Volksvertretung gilt hingegen der bis 979 zurückverfolgbare Tynwald der Isle of Man.