The Alþingi, anglicised variously as Althing or Althingi, is the national parliament (literally, "(the) all-thing", or general assembly) of Iceland. The Althingi is the oldest parliamentary institution in the world still extant. It 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, and 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.

The constitution of Iceland provides for six electoral constituencies with the possibility of an increase to seven. The constituency boundaries are fixed by legislation. Each constituency elects nine members. In addition, each party is allocated seats based on its proportion of the overall national vote in order that the number of members in parliament for each political party should be more or less proportional to its overall electoral support. A party must have won at least five per cent of the national vote in order to be eligible for these proportionally distributed seats. Political participation in Iceland is very high: usually over 85 per cent of the electorate casts a ballot (87.7% in 2003). The current president of the Althing is Ásta Ragnheiður Jóhannesdóttir.