- January 2 - The first religious radio broadcast (KDKA AM in Pittsburgh, Pennsylvania)
- January 2 - Spanish liner Santa Isabel sinks off Villa Garcia - 244 dead
- January 2 - DeYoung Museum in Golden Gate Park San Francisco opens.
- January 20 - Republic of Turkey is declared
- January 20 - Royal Navy K-boat K5 sinks in the English Channel with all 56 hands
- February 25 - The Democratic Republic of Georgia is occupied by the Soviet Russia.
- February 28 - Russian sailors rebel in Kronstadt - At March 17 Red Army crushes the rebellion and number of sailors flee to Finland
- March 1 - the city Kiryu, located in Gunma, Japan, is founded.
- March 8 - Spanish Premier Eduardo Dato is assassinated while exiting the parliament building in Madrid.
- March 13 - Mongolia declares its independence from China
- March 17 - Marie Stopes opens the first Birth Control Clinic in London, England
- March 18 - The second Peace of Riga between Poland and Soviet Union ending Polish-Soviet war. Despite the recent Polish successes, Soviets annex Ukraine and Belarus.
- April 11 - The Emirate of Transjordan is created.
- April 14 - In Britain, labor unions for mining, railway and transportation workers call for a strike - government threatens to call in the army
- April 24 - Referendum in Tyrol supports joining to Germany
- May 1-7 - Riots in Palestine of May, 1921
- May 6 - General strike begins in Norway
- May 5 - Only 13 spectators follow the soccer match between Leicester City and Stockport county
- May 8 - Death penalty abolished in Sweden
- May 14 - 17 - Violent anti-European riots in Cairo and Alexandria
- May 19 - The Emergency Quota Act passes the United States Congress establishing national quotas on immigration.
- May 24 - Elections are held for the first time for the new Northern Ireland Parliament.
- June 1 - Tulsa Race Riot of 1921: A race riot in Tulsa, Oklahoma kills 85 people.
- June 26 - In Britain, rain ends 100 days of drought
- July 1 - Coal strike ends in England
- July 11 - The Irish War of Independence comes to an end when a truce is signed between the British Government and the Irish forces.
- July 18 - The first BCG vaccination against tuberculosis
- July 22 - Irish Truce declared in Britain
- July 26 - US President Warren G. Harding receives Princess Fatima of Afghanistan - and Stanley Clifford Weyman...
- July 27 - Researchers at the University of Toronto led by biochemist Frederick Banting announce the discovery of the hormone insulin.
- August - The United States formally ends World War I, declaring a peace with Germany
- August 11 - 35 degree Celsius in Breslau - heat wave continues elsewhere in the Europe as well
- August 23 - Feisal I becomes the king of Egypt
- August 24 - Airship ZR 2 explodes during a test flight near Hull, England - 41 dead
- August 26 - Rising prizes cause riots in Munich
- August 29 - Assassination of German politician Matthias Erzberger causes the government to declare martial law
- September 1 - Poplar Strike in London - 9 members of Poplar borough council are arrested
- September 7 - In Atlantic City, New Jersey, the first Miss America Pageant is held.
- September 12 - Lotta Svärd founded in Finland
- September 21 - Chemical plant explodes in Ludwigshafen, Germany - 535-561 dead
- October 21 - Peace conference between Irish and United Kingdom begins in London
- October 24 - Spanish army defeats rifkabyls
- November 9 - Riots in Reykjavík - most of the small police force is injured
- November 11 - During an Armistice Day ceremony at Arlington National Cemetery, the Tomb of the Unknowns is dedicated by US President Warren G. Harding.
- December 1 - Rising prizes cause riots in Vienna
- December 16 - The Anglo-Irish Treaty establishing the Irish Free State is signed in London. See Ireland/History
- December 29 - William Lyon Mackenzie King becomes Canada's tenth prime minister.
- January 5 - Friedrich Dürrenmatt, writer (d. 1990)
- January 5 - Grand Duke Jean of Luxembourg
- January 6 - Louis Harris, pollster
- January 10 - Rodger Ward, auto racer, two-time Indianapolis 500 winner (d. 2004)
- January 19 - Patricia Highsmith, author (d. 1995)
- January 20 - Bernt Engelmann, author (d. 1994)
- January 27 - Donna Reed, actress (d. 1986)
- January 31 - Carol Channing, actress
- January 31 - Mario Lanza, singer, actor (d. 1959)
- February 4 - Betty Friedan, feminist
- February 11 - Eva Gabor, Hungarian actress (d. 1996)
- February 11 - Lloyd Bentsen, American politician.
- February 14 - Hugh Downs, game show host, journalist
- February 25 - Pierre Laporte, statesman, murdered by FLQ terrorists (d. 1970)
- March 1 - Jack Clayton, film director
- March 1 - Richard Wilbur, American poet
- March 1 - Terence Cardinal Cooke, tenth bishop (seventh archbishop) of the Roman Catholic diocese of New York († 1983)
- March 3 - Paul Guimard, French writer (d. 2004)
- March 8 - Cyd Charisse, actress, dancer
- March 12 - Giovanni Agnelli, manager of Fiat (d. 2003)
- March 12 - Gordon MacRae, singer, actor (d. 1986)
- March 20 - Rudolf Noelte, film director (d. 2002)
- March 20 - Sister Rosetta Tharpe, singer (d. 1973)
- March 25 - Simone Signoret, actress (d. 1985)
- March 28 - Dirk Bogarde, actor (d. 1999)
- April 15 - Georgi Beregovoi, cosmonaut (d. 1995)
- April 23 - Warren Spahn, Baseball Hall of Famer (d. 2003)
- May 2 - Satyajit Ray, Indian filmmaker (d. 1992)
- May 6 - Erich Fried, author (d. 1988)
- May 9 - Sophie Scholl, resistance fighter with White Rose in Nazi Germany (d. 1943)
- May 11 - Hildegard Hamm-Brücher, politician
- May 12 - Joseph Beuys, artist (d. 1986)
- May 12 - Farley Mowat, writer, naturalist
- May 19 - Karel van het Reve, Dutch writer (d. 1999)
- May 20 - Wolfgang Borchert, lyricist, narrator and dramatist (d. 1947)
- May 20 - Hal Newhouser, Baseball Hall of Famer (d. 1998)
- May 21 - Andrei Sakharov, physicist, human rights activist, recipient of the Nobel Prize (d. 1989)
- May 23 - James Blish, US science fiction author (d. 1975)
- May 28 - Heinz G. Konsalik, author (d. 1999)
- June 1 - Nelson Riddle, bandleader (d. 1985)
- June 10 - Prince Philip, Duke of Edinburgh
- June 26 - Violette Szabo, SOE agent, WW II heroine (d. 1945)
- July 4 - Tibor Varga, violinist, conductor and pedagogue
- July 17 - Hannah Szenes, WW II heroine executed by the Nazis (d. 1944)
- July 30 - William Chester Owens, WW II plane captain of USN Ace Capt. David McCampbell
- October 13 - Yves Montand, French singer and actor (d. 1991)
- October 18 - Jesse Helms, former US Senator from North Carolina
- October 19 - Gunnar Nordahl, Swedish footballer, 2nd highest Serie A goalscorer of all-time (d. 1995)
- October 25 - King Michael of Romania
- November 5 - Princess Fawzia of Egypt
- November 22 - Rodney Dangerfield, actor, comedian (d. 2004)
- November 23 - Fred Buscaglione, Italian singer and actor
- November 24 - Yoshiko Uchida, Japanese-American writer
- December 26 - Steve Allen, actor, composer, comedian, author (d. 2000)