The prime minister of Northern Ireland was the head of the Government of Northern Ireland between 1921 and 1972. No such office was provided for in the Government of Ireland Act 1920; however, the Lord Lieutenant of Ireland, as with governors-general in other Westminster systems such as in Canada, chose to appoint someone to head the executive even though no such post existed in statute law. The office-holder assumed the title prime minister to draw parallels with the prime minister of the United Kingdom. On the advice of the new prime minister, the lord lieutenant then created the Department of the Prime Minister. The office of Prime Minister of Northern Ireland was suspended in 1972 and then abolished in 1973, along with the contemporary government, when direct rule of Northern Ireland was transferred to London.

The Government of Ireland Act 1920 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.