The Yemeni unification took place on 22 May 1990, when the People's Democratic Republic of Yemen (South Yemen) and the Yemen Arab Republic (North Yemen) united, forming the Republic of Yemen.

North Yemen became an independent Kingdom in the context of the dissolution of the Ottoman Empire in November 1918. Aden, in South Yemen, was administered as a British protectorate and in 1937 became a British colony in its own right. The larger part of South Yemen was a British protectorate, effectively under colonial control. In one of the many proxy conflicts of the Cold War, a South Yemeni insurgency (with the support and backing of the Soviet Union) led by two nationalist parties revolted, causing the United Kingdom to unify the area and in 1967 to withdraw from its former colony.

Following the North Yemen Civil War, the north overthrew the monarchy and established a Nasserist republican government led by a military junta that included tribal representatives. It enjoyed modest oil revenues and remittances from its citizens working in the oil-rich Arab states of the Persian Gulf. Its population in the 1980s was estimated at 12 million as opposed to 3 million in South Yemen.

South Yemen developed as a mostly secular society ruled first by the National Liberation Front, which later morphed into the ruling Yemeni Socialist Party. The only avowedly communist nation in the Middle East, South Yemen received significant foreign aid and other assistance from the Soviets.

In October 1972, fighting erupted between north and south; North Yemen was supplied by Saudi Arabia, and South Yemen was supplied by the Soviet Union. Fighting was short-lived, and the conflict led to the 28 October 1972 Cairo Agreement, which set forth a plan to unify the two countries.

Fighting broke out again in February and March 1979, with South Yemen allegedly supplying aid to rebels in the north by the National Democratic Front and crossing the border. Southern forces made it as far as the city of Taiz before withdrawing. Again, North Yemen was supported by anticommunist Saudi Arabia and Taiwan, and secretly in the name of the Royal Saudi Air Force from 1979 to 1990. This conflict was also short-lived.

In the late 1980s, oil exploration near the border between the two nations – the Marib Governorate in the North and the Shabwah Governorate in the South – spurred interest in developing agreements to exploit resources there and lift both nations' economies. In May 1988, the two governments came to an understanding that considerably reduced tensions, including agreements to renew discussions concerning unification, to establish a joint oil exploration area along their undefined border, now called the Joint Investment Area, by the Hunt Oil Company and Exxon. The same month, they formed the Yemeni Company for Investment in Mineral and Oil Resources (YCIMOR).

In November 1989, Ali Abdullah Saleh of North Yemen and Ali Salem al Beidh of South Yemen jointly accepted a draft unity constitution originally drawn up in 1981, which included a demilitarized border and border passage by Yemenis on the sole basis of a national identification card and a capital city in Sanaa.