The Government of Ireland Act 1920 (10 & 11 Geo. 5. c. 67) was an act of the Parliament of the United Kingdom. The Act's long title was "An Act to provide for the better government of Ireland"; it is also known as the Fourth Home Rule Bill or (inaccurately) as the Fourth Home Rule Act and informally known as the Partition Act. The Act was intended to partition Ireland into two self-governing polities: the six north-eastern counties were to form "Northern Ireland", while the larger part of the country was to form "Southern Ireland". Both territories were to remain part of the United Kingdom of Great Britain and Ireland and provision was made for their future reunification through a Council of Ireland. The Act was passed by the British Parliament in November 1920, received royal assent in December and came into force on 3 May 1921.

The smaller Northern Ireland was duly created with a devolved government and remained in the UK. The larger Southern Ireland was not recognised by most of its citizens, who instead recognised the self-declared Irish Republic in the ongoing Irish War of Independence. The conflict resulted in the Anglo-Irish Treaty in December 1921. Under the treaty, Ireland would leave the UK (with the option for Northern Ireland to opt out and remain in the UK, which it immediately did) in December 1922 and become the Irish Free State, which would later evolve into today's Republic of Ireland. The institutions set up under this Act for Northern Ireland continued to function until they were suspended by the British parliament in 1972 as a consequence of the Troubles.

The remaining provisions of the Act still in force in Northern Ireland were repealed under the terms of the 1998 Good Friday Agreement.

Various attempts had been made to give Ireland limited regional self-government, known as Home rule, in the late 19th and early 20th centuries.

The First Home Rule Bill of 1886 was defeated in the House of Commons because of a split in the Liberal Party over the principle of Home Rule, while the Second Home Rule Bill of 1893, having been passed by the Commons was vetoed by the House of Lords. The Third Home Rule Bill introduced in 1912 by the Irish Parliamentary Party could no longer be vetoed after the passing of the Parliament Act 1911 which removed the power of the Lords to veto bills. They could merely be delayed for two years.

Because of the continuing threat of civil war in Ireland, King George V called the Buckingham Palace Conference in July 1914 where Irish Nationalist and Unionist leaders failed to reach agreement. Controversy continued over the rival demands of Irish Nationalists, backed by the Liberals (for all-Ireland home rule), and Irish Unionists, backed by the Conservatives, for the exclusion of most or all of the province of Ulster. In an attempt at compromise, the British government put forward an amending bill, which would have allowed for Ulster to be temporarily excluded from the working of the Act; this failed to satisfy either side and the stalemate continued until overtaken by the outbreak of World War I. A few weeks after the British entry into the war, the Act received Royal Assent, while the amending bill was abandoned. However, the Suspensory Act 1914 (which received Royal Assent on the same day) meant that implementation would be suspended for the duration of what was expected to be only a short European war.

During the Great War, Irish politics moved decisively in a different direction. Several events, including the Easter Rising of 1916, the subsequent reaction of the British Government and the Conscription Crisis of 1918, had altered the state of Irish politics and contributed to Sinn Féin becoming the dominant voice of Irish nationalism. Sinn Féin, standing for 'an independent sovereign Ireland', won 73 of the 105 parliamentary seats on the island in the 1918 general election. Its elected members established their own parliament, Dáil Éireann, which declared the country's independence as the Irish Republic. Dáil Éireann, after a number of meetings, was declared illegal in September 1919 by the Lord Lieutenant of Ireland, Viscount French.

When the Act became law on 23 December 1920 it was already out of touch with realities in Ireland. The long-standing demand for home rule had been replaced among nationalists by a demand for complete independence (an Irish Republic). The Republic's army was waging the Irish War of Independence against British rule, which had reached a nadir in late 1920.