Events from the year 1921 in the United States.
- President: Woodrow Wilson (D-New Jersey) (until March 4), Warren G. Harding (R-Ohio) (starting March 4)
- Vice President: Thomas R. Marshall (D-Indiana) (until March 4), Calvin Coolidge (R-Massachusetts) (starting March 4)
- Chief Justice: Edward Douglass White (Louisiana) (until May 19), William Howard Taft (Ohio) (starting July 11)
- Speaker of the House of Representatives: Frederick H. Gillett (R-Massachusetts)
- Senate Majority Leader: Henry Cabot Lodge (R-Massachusetts)
- Congress: 66th (until March 4), 67th (starting March 4)
- January 1 – In American football, the University of California defeats Ohio State 28–0 in the Rose Bowl.
- January 2 – The first religious radio broadcast is heard over station (KDKA AM in Pittsburgh, Pennsylvania).
- January 21 – The full-length silent comedy-drama film The Kid, written, produced, directed by and starring Charlie Chaplin (in his Tramp character), with Jackie Coogan, is released.
- March 4 – Warren G. Harding is sworn in as the 29th President of the United States.
- April 20 – Ferenc Molnár's play Liliom is first produced on Broadway in English.
- May 19 – The Emergency Quota Act passes the U.S. Congress, establishing national quotas on immigration. Because this drastically limits immigration from Eastern Europe, Jews emigrating from there begin to prefer Palestine as a destination rather than the U.S.
- May 27 – First victim of the Osage Indian murders is discovered in Osage County, Oklahoma.
- May 31 – Tulsa Race Riot: The official death toll is 39, but later investigations suggest the actual toll may be much higher.
- July 2 – U.S. President Warren Harding signs a joint congressional resolution declaring an end to America's state of war with Germany, Austria and Hungary.
- July 11 – Former President of the United States William Howard Taft is sworn in as Chief Justice of the United States, making him the only person ever to hold both positions.
- July 14 – A Massachusetts jury finds Nicola Sacco and Bartolomeo Vanzetti guilty of first degree murder following a widely publicized trial.
- July 26 – U.S. President Warren G. Harding receives Princess Fatima of Afghanistan and Stanley Clifford Weyman.
- August – The United States formally ends World War I, declaring a peace with Germany.
- August 5 – The first radio baseball game is broadcast; Harold Arlin announces the Pirates-Phillies game from Forbes Field over Westinghouse KDKA, in Pittsburgh.
- August 11 – Franklin D. Roosevelt's paralytic illness strikes while he is vacationing on Campobello Island; he is diagnosed with polio.
- August 25–September 2 – An uprising of striking coal miners in West Virginia leads to the Battle of Blair Mountain.
- September 5 – Popular comedian Roscoe "Fatty" Arbuckle attends a party at the St. Francis Hotel, San Francisco, during which actress Virginia Rappe is fatally injured; although he is eventually acquitted of rape and manslaughter, the scandal derails his career.
- September 7 – In Atlantic City, New Jersey, the first Miss America Pageant is held.
- September 8 – Sixteen-year-old Margaret Gorman wins the Atlantic City Pageant's Golden Mermaid trophy; pageant officials later dub her the first Miss America.
- September 13 – White Castle hamburger restaurant opens in Wichita, Kansas, the foundation of the world's first fast food chain.
- October 8 – The first Sweetest Day is staged in Cleveland, Ohio.
- October 26 – The Chicago Theatre, the oldest surviving grand movie palace, opens.
- October 29 – 1921 Centre vs. Harvard football game: Centre College's football team, led by quarterback Bo McMillin, defeats Harvard University 6–0 to snap Harvard's five-year winning streak. For decades afterward, this is called "football's upset of the century."
- November 11 – During an Armistice Day ceremony at Arlington National Cemetery, the Tomb of the Unknowns is dedicated by U.S. President Warren G. Harding.
- December 13 – In the Four Power Treaty on Insular Possessions, Japan, the United States, United Kingdom and France agree to recognize the status quo in the Pacific.
- Simon Rodia begins construction of the Watts Towers in Los Angeles.
- The central tower is added to the De Young (museum) museum in Golden Gate Park, San Francisco.
- The Tau Epsilon Chi Jewish high school sorority is founded in Atlantic City, New Jersey.
- Lochner era (c. 1897–c. 1937)
- U.S. occupation of Haiti (1915–1934)
- Prohibition (1919–1933)
- Depression of 1920–21 (1920–1921)
- Roaring Twenties (1920–1929)
- January 19 – Patricia Highsmith, crime fiction writer (died 1995 in Switzerland)
- January 29
- January 31
- February 4 – Betty Friedan, feminist author (died 2006)
- February 11 – Lloyd Bentsen, U.S. Senator from Texas from 1971 to 1993 (died 2006)
- February 17 – Duane Gish, biochemist and academic (died 2013)
- March 1 – Richard Wilbur, poet laureate (died 2017)
- March 27 – Tom Bevill, U.S. Representative from Alabama from 1967 to 1997 (died 2005)
- March 31 – Peggy Rea, television actress (died 2011)
- April 21 – John R. Huizenga, nuclear physicist (died 2014)
- April 23 – Judy Agnew, Second Lady of the United States as wife of Spiro Agnew (died 2012)
- May 1 – Boo Morcom, pole vaulter and jumper (died 2012)
- May 9
- June 7 – Dorothy Ruth, horse breeder and author (died 1989)
- June 9 – Forrest Bird, biomedical engineer (died 2015)
- June 13 – Nancy Warren, baseball pitcher (died 2001)
- June 15 – James Emanuel, African American poet and scholar (died 2013)
- June 17 – Tony Scott, jazz clarinetist (died 2007)
- June 19 – Howell Heflin, U.S. Senator from Alabama from 1979 to 1997 (died 2005)
- July 4 – Philip Rose, actor, playwright and theatrical producer (died 2011)
- July 6
- July 18 – John Glenn, first U.S. astronaut to orbit Earth and U.S. Senator from Ohio from 1974 to 1999 (died 2016)
- July 22 – William V. Roth, Jr., U.S. Senator from Delaware from 1971 to 2001 (died 2003)
- August 4 – Herb Ellis, jazz guitarist (died 2010)
- August 19 – Gene Roddenberry, television producer (died 1991)
- August 23 – Kenneth Arrow, economist, recipient of the Nobel Memorial Prize in Economic Sciences in 1972 (died 2017)
- August 26
- September 3 – Henry Bellmon, U.S. Senator from Oklahoma from 1969 to 1981 (died 2009)
- October 5 – Mahlon Hoagland, biochemist (died 2009)
- October 21 – Victor A. McKusick, "father of genetic medicine" (died 2008)
- November 3 – Charles Bronson, film actor (died 2003)
- November 6 – James Jones, novelist (died 1977)
- November 20 – Dan Frazer, actor (died 2011)
- November 22 – Rodney Dangerfield, born Jacob Rodney Cohen, comedian (died 2004)
- November 29 – Jackie Stallone, born Jacqueline Labofish, astrologer, dancer, wrestling promoter and mother of Sylvester Stallone
- December 3
- December 21 – Robert Lipshutz, lawyer and politician, 17th White House Counsel (died 2010)
- December 26 – Steve Allen, television host (died 2000)
- March 8 – Thomas H. Paynter, U.S. Senator from Kentucky from 1907 to 1913 (born 1851)
- March 29
- April 21 – Tom O'Brien, baseball player (born 1860)
- May 19 – Edward Douglass White, 9th Chief Justice of the United States from 1910 to 1921, Associate Justice of the Supreme Court from 1894 to 1910 and U.S. Senator from Louisiana from 1891 to 1894 (born 1845)
- May 26 – Donald Evans, poet, publisher, music critic and journalist (born 1884)
- June 12 – Murphy J. Foster, U.S. Senator from Louisiana from 1901 to 1913 (born 1849)
- June 16 – William E. Mason, U.S. Senator from Illinois from 1897 to 1903 (born 1850)
- September 9 – Virginia Rappe, model and silent film actress (born 1895)
- October 12 – Philander C. Knox, United States Attorney General from 1901 to 1904 and United States Secretary of State from 1909 to 1913 (born 1853)
- October 25 – Bat Masterson, gunfighter (born 1853)
- December 12 – Henrietta Swan Leavitt, astronomer (born 1868)
- Nat Love, African American cowboy (born 1854)
- Media related to 1921 in the United States at Wikimedia Commons