Black September (Arabic: أيلول الأسود; Aylūl Al-Aswad), also known as the Jordanian Civil War, was a conflict fought in the Hashemite Kingdom of Jordan between the Jordanian Armed Forces (JAF), under the leadership of King Hussein, and the Palestine Liberation Organization (PLO), under the leadership of Yasser Arafat, primarily between 16 and 27 September 1970, with certain aspects of the conflict continuing until 17 July 1971.

After Jordan lost control of the West Bank to Israel in 1967, Palestinian fighters known as fedayeen moved their bases to Jordan and stepped up their attacks on Israel and Israeli-occupied territories. One Israeli retaliation on a PLO camp based in Karameh, a Jordanian town along the border with the West Bank, developed into a full-scale battle. The perceived joint Jordanian-Palestinian victory against Israel during the 1968 Battle of Karameh led to an upsurge in Arab support for the fedayeen in Jordan, in both new recruits and financial aid. The PLO's strength in Jordan grew, and by the beginning of 1970, groups within the PLO had begun openly calling for the overthrow of the Hashemite monarchy.

Acting as a state within a state, the fedayeen disregarded local laws and regulations, and even attempted to assassinate King Hussein twice—leading to violent confrontations between them and the Jordanian Army in June 1970. Hussein wanted to oust them from the country, but hesitated to strike because he did not want his enemies to use it against him by equating Palestinian fighters with civilians. PLO actions in Jordan culminated in the Dawson's Field hijackings incident of 6 September, in which the PFLP hijacked three civilian aircraft and forced their landing in Zarqa, taking foreign nationals as hostages, and later blowing up the planes in front of journalists from around the world. Hussein saw this as intolerable, and ordered the army to take action.

On 17 September, the Jordanian Army surrounded cities with significant PLO presence including Amman and Irbid, and began shelling Palestinian refugee camps where the fedayeen were established. The next day, forces from the Syrian Army, with Palestine Liberation Army markings, intervened in support of the fedayeen and advanced towards Irbid which the fedayeen had occupied and declared to be a "liberated" city. On 22 September, the Syrians withdrew from Irbid after suffering heavy losses in an air-ground offensive launched by the Jordanians. Mounting pressure by Arab countries (such as Iraq) led Hussein to halt the fighting. On 13 October he signed an agreement with Arafat to regulate the fedayeen's presence in Jordan. However, the Jordanian military attacked again in January 1971, and the fedayeen were driven out of the cities, one by one, until 2,000 fedayeen surrendered after being surrounded in a forest near Ajloun on 17 July, marking the end of the conflict.

Jordan allowed the fedayeen to leave for Lebanon via Syria, and the fedayeen later participated in the 1975 Lebanese Civil War. The Black September Organization was founded after the conflict to carry out reprisals against Jordanian authorities, and the Organization's first noted attack was the assassination of Jordanian Prime Minister Wasfi Tal in 1971 who had commanded parts of the operations that expelled the fedayeen. The Organization then shifted to attacking Israeli targets, including the highly publicized Munich massacre of Israeli athletes. Even though Black September did not reflect a Jordanian-Palestinian divide, as there were Palestinians and Jordanians on both sides of the conflict, it paved the way for such a divide to emerge subsequently.