The Black September Organization (BSO; Arabic: منظمة أيلول الأسود, romanized: Munaẓẓamat Aylūl al-Aswad) was a Palestinian militant organization, which was founded in September 1970. Besides other actions, the group was responsible for the assassination of the Jordanian Prime Minister Wasfi Tal, and the Munich massacre, in which eleven Israeli athletes and officials were kidnapped and killed, as well as a West German policeman dying, during the 1972 Summer Olympics in Munich, their most publicized event. These attacks led to the creation or specialization of permanent counter-terrorism forces in many European countries.

The group's name is derived from the Black September conflict which began on 16 September 1970, when King Hussein of Jordan declared military rule in response to fedayeen attempting to seize his kingdom – resulting in the deaths and expulsion of thousands of Palestinian fighters from Jordan. The BSO began as a small cell of Fatah men determined to take revenge upon King Hussein and the Jordanian Armed Forces. Recruits from the PFLP, as-Sa'iqa, and other groups also joined.

Initially, most of its members were dissidents within Fatah who had been close to Abu Ali Iyad, the commander of Fatah forces in northern Jordan who continued to fight the Jordanian Army after the PLO leadership withdrew. He was killed, allegedly through execution, by Jordanian forces on 23 July 1971. It was alleged by them that the Jordanian Prime Minister at the time, Wasfi Tal, was personally responsible for his torture and death.

There is disagreement among historians, journalists, and primary sources about the nature of the BSO and the extent to which it was controlled by Fatah, the Palestine Liberation Organization (PLO) faction controlled at the time by Yasser Arafat.

In his book Stateless, Salah Khalaf (Abu Iyad), Arafat's chief of security and a founding member of Fatah, wrote that: "Black September was not a terrorist organization, but was rather an auxiliary unit of the resistance movement, at a time when the latter was unable to fully realize its military and political potential. The members of the organization always denied any ties between their organization and Fatah or the PLO."

The denial described in Abu Iyad's claim was mutual: according to a 1972 article in the Jordanian newspaper Ad-Dustour, Mohammed Daoud Oudeh, also known as Abu Daoud, a BSO operative and former senior PLO member, told Jordanian police: "There is no such organization as Black September. Fatah announces its own operations under this name so that Fatah will not appear as the direct executor of the operation." A March 1973 document released in 1981 by the U.S. State Department seemed to confirm that Fatah was Black September's parent organization.

According to American journalist John K. Cooley, the BSO represented a "total break with the old operational and organizational methods of the fedayeen. Its members operated in air-tight cells of four or more men and women. Each cell's members were kept purposely ignorant of other cells. Leadership was exercised from outside by intermediaries and 'cut-offs' [sic]", though there was no centralized leadership.

Cooley writes that many of the cells in Europe and around the world were made up of Palestinians and other Arabs who had lived in their countries of residence as students, teachers, businessmen, and diplomats for many years. Operating without a central leadership, it was a "true collegial direction". The cell structure and the need-to-know operational philosophy protected the operatives by ensuring that the apprehension or surveillance of one cell would not affect the others. The structure offered plausible deniability to the Fatah leadership, which was careful to distance itself from Black September operations.