| 108001 | September 21 | Event | 1964 – The North American XB-70 Valkyrie, the world's first Mach 3 bomber, makes its maiden flight from Palmdale, California. |
| 108002 | September 21 | Event | 1965 – The Gambia, Maldives and Singapore are admitted as members of the United Nations. |
| 108003 | September 21 | Event | 1971 – Bahrain, Bhutan and Qatar join the United Nations. |
| 108004 | September 21 | Event | 1972 – Philippine President Ferdinand Marcos signs Proclamation № 1081, placing the entire country under martial law and marking the beginning of his authoritarian rule. |
| 108005 | September 21 | Event | 1976 – Orlando Letelier is assassinated in Washington, D.C. He is a member of the Chilean socialist government of Salvador Allende, overthrown in 1973 by Augusto Pinochet. |
| 108006 | September 21 | Event | 1976 – Seychelles joins the United Nations. |
| 108007 | September 21 | Event | 1977 – A nuclear non-proliferation pact is signed by 15 countries, including the United States and the Soviet Union. |
| 108008 | September 21 | Event | 1981 – Belize is granted full independence from the United Kingdom. |
| 108009 | September 21 | Event | 1981 – Sandra Day O'Connor is unanimously approved by the U.S. Senate as the first female Supreme Court justice. |
| 108010 | September 21 | Event | 1984 – Brunei joins the United Nations. |
| 108011 | September 21 | Event | 1991 – Armenia is granted independence from Soviet Union. |
| 108012 | September 21 | Event | 1993 – Russian President Boris Yeltsin suspends parliament and scraps the then-functioning constitution, thus triggering the Russian constitutional crisis of 1993. |
| 108013 | September 21 | Event | 1999 – Chi-Chi earthquake occurs in central Taiwan, leaving about 2,400 people dead. |
| 108014 | September 21 | Event | 2001 – Deep Space 1 flies within 2,200 km of Comet Borrelly. |
| 108015 | September 21 | Event | 2001 – America: A Tribute to Heroes is broadcast by over 35 network and cable channels, raising over $200 million for the victims of the September 11 attacks. |
| 108016 | September 21 | Event | 2003 – Galileo mission is terminated by sending the probe into Jupiter's atmosphere, where it is crushed by the pressure at the lower altitudes. |
| 108017 | September 21 | Event | 2005 – Hurricane Rita becomes the third most intense hurricane (dropped to 4th on October 19, 2005). |
| 108018 | September 21 | Event | 2013 – al-Shabaab Islamic militants attack the Westgate shopping mall in Nairobi, Kenya, killing at least 67 people. |
| 108019 | September 21 | Birth | 1051 – Bertha of Savoy (d. 1087) |
| 108020 | September 21 | Birth | 1411 – Richard of York, 3rd Duke of York, English politician, Lord Protector of England (d. 1460) |
| 108021 | September 21 | Birth | 1415 – Frederick III, Holy Roman Emperor (d. 1493) |
| 108022 | September 21 | Birth | 1428 – Jingtai Emperor of China (d. 1457) |
| 108023 | September 21 | Birth | 1433 – Guillaume Fichet, French scholar and academic (d. 1480) |
| 108024 | September 21 | Birth | 1452 – Girolamo Savonarola, Italian priest and philosopher (d. 1498) |
| 108025 | September 21 | Birth | 1559 – Cigoli, Italian painter and architect (d. 1613) |
| 108026 | September 21 | Birth | 1629 – Philip Howard, English cardinal (d. 1694) |
| 108027 | September 21 | Birth | 1645 – Louis Jolliet, Canadian explorer (d. 1700) |
| 108028 | September 21 | Birth | 1706 – Polyxena of Hesse-Rotenburg (d. 1735) |
| 108029 | September 21 | Birth | 1756 – John Loudon McAdam, Scottish engineer (d. 1836) |
| 108030 | September 21 | Birth | 1758 – Christopher Gore, American soldier, lawyer, and politician, 8th Governor of Massachusetts (d. 1827) |