The Kingdom of Iceland (Icelandic: Konungsríkið Ísland; Danish: Kongeriget Island) was a constitutional monarchy that existed through the Act of Union with Denmark signed on 1 December 1918. It lasted until 17 June 1944 when a national referendum established the Republic of Iceland in its place.

Iceland had been under the control of the Crown of Denmark since 1380, although formally it had been a Norwegian possession until 1814. In 1874, one thousand years after the first acknowledged settlement, Denmark granted Iceland home rule. The constitution, written the same year, was revised in 1903 and the extent of Iceland's home rule increased in 1904. A minister for Icelandic affairs, residing in Reykjavík, was made responsible to the Althing, the Icelandic parliament.

On 1 December 1918, the Act of Union, an agreement with Denmark, recognized Iceland as a fully sovereign state in personal union with Denmark through a common monarch. The Kingdom of Iceland established its own flag and coat of arms and asked that Denmark represent its foreign affairs and defence interests. The Act would be reviewed in 1940 and could be revoked three years later if agreement to continue it could not be reached.

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