Alþingishúsið is a classical 19th century structure which stands by Austurvöllur in central Reykjavík, Iceland. It houses Alþingi, the Icelandic parliament. The building was designed by Danish architect Ferdinand Meldahl and built using hewn dolerite from 1880 to 1881.
Alþingishúsið has also housed the Icelandic National Library and Antiquaries Collection, and later the Icelandic National Gallery. The University of Iceland used the first floor of the house from 1911 to 1940, and the President of Iceland had his offices in the building until 1973.