This article is about the year 1821.
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1821 (MDCCCXXI) was a common year starting on Monday (dominical letter G) of the Gregorian calendar and a common year starting on Saturday (dominical letter B) of the Julian calendar, the 1821st year of the Common Era (CE) and Anno Domini (AD) designations, the 821st year of the 2nd millennium, the 21st year of the 19th century, and the 2nd year of the 1820s decade. Note that the Julian day for 1821 is 12 calendar days difference, which continued to be used from 1582 until the complete conversion of the Gregorian calendar was entirely done in 1929.
- July 10 – The United States takes possession of its newly bought territory of Florida from Spain.
- July 19 – George IV is crowned king of the United Kingdom of Great Britain and Ireland.
- July 28 – Peru declares independence from Spain.
- August 10 – Missouri is admitted as the 24th U.S. state (see History of Missouri).
- August 19 – Navarino massacre: Greek rebels massacre 3,000 inhabitants of the city of Navarino.
- August 21 – Jarvis Island is discovered by the crew of the Eliza Frances.
- August 24 – Treaty of Córdoba signed in Córdoba, Veracruz, Mexico, ratifying the Plan of Iguala of February 24 for Mexico's independence from Spain and, although not recognised by the latter, effectively concludes the Mexican War of Independence.
- September 4
- September 7 – The Republic of Gran Colombia (a federation covering much of present-day Venezuela, Colombia, Panama, and Ecuador) is established, with Simón Bolívar as the founding President and Francisco de Paula Santander as vice president.
- September 15 – Guatemala, El Salvador, Honduras, Nicaragua and Costa Rica gain independence from Spain by the Act of Independence of Central America. On October 29, the newly independent First Mexican Empire proposes that Guatemala should merge with it.
- September 18 – Amherst College is founded in Massachusetts.
- September 23 – Greek War of Independence: Siege of Tripolitsa ends when Greek rebels capture the city of Tripoli, Greece.
- September 27 – The Army of the Three Guarantees enters Mexico City and the Declaration of Independence of the Mexican Empire from Spain is proclaimed, following the Mexican War of Independence.
The oath in Agia Lavra, March 25, 1821
The coronation banquet for George IV
- January 8
- January 16 – John C. Breckinridge, United States Vice president and Confederate Secretary of State (d. 1875).
- February 3 – Elizabeth Blackwell, first American female physician (d. 1910).
- February 11
- February 17 – Lola Montez, Irish Spanish dancer and royal mistress (d. 1861)
- February 19 – August Schleicher, German linguist (d. 1868)
- March 1 – Joseph Hubert Reinkens, German Old Catholic bishop (d. 1896)
- March 9 – John Watts de Peyster, American author, philanthropist, and soldier (d. 1907)
- March 12 – Sir John Abbott, 3rd Prime Minister of Canada (d. 1893)
- March 15 – William Milligan, Scottish theologian (d. 1892)
- April 3 – Fr. Thomas Pelham Dale English mystic (d. 1892)
- April 9 – Charles Baudelaire, French poet and writer (d. 1867)
- May 6 – Edmund Colhoun, American admiral (d. 1897)
- May 8 – William Henry Vanderbilt, American entrepreneur (d. 1885)
- May 16 – Pafnuty Chebyshev, Russian mathematician (d. 1894)
- May 17 – Sebastian Kneipp, German naturopath (d. 1897)
- June 16 – Old Tom Morris, Scottish golfer (d. 1908)
- June 26 – Bartolomé Mitre, Argentine statesman, military figure, and author, sixth President of Argentina (d. 1906)
- July 1 – Anatole Jean-Baptiste Antoine de Barthélemy, French archaeologist (d. 1904)
- July 2 – Charles Tupper, 6th Prime Minister of Canada (d. 1915)
- July 9
- July 16 – Mary Baker Eddy, American founder of Christian Science (d. 1910)
- July 18 – Pauline Viardot, French mezzo-soprano and composer (d. 1910)
- July 24 – William Poole, infamous member of New York City's Bowery Boys gang (d. 1855)
- July 27 – George H. Cooper, United States Navy admiral (d. 1891)
- August 10 – Jay Cooke, American financier (d. 1905)
- August 21 – Louis Vuitton, French fashion designer (d. 1892)
- August 31 – Hermann von Helmholtz, German physician and physicist (d. 1894)
- September 21 – Andrei Alexandrovich Popov, Russian admiral (d. 1898)
- September 28 – Jonathan Clarkson Gibbs, African-American minister and politician (d. 1874)
- October 13 – Rudolf Virchow, German physician, pathologist, biologist, and politician (d. 1902)
- November 11 – Fyodor Dostoyevsky, Russian writer (d. 1881)
- November 30 – Frederick Temple, Archbishop of Canterbury (d. 1902)
- December 1 – John M. B. Clitz, American admiral (d. 1897)
- December 12 – Gustave Flaubert, French writer (d. 1880)
- December 24 – Gabriel García Moreno, former President of Ecuador (d. 1875)
- December 25 – Clara Barton, first president of American Red Cross (d. 1912)
- January 4 – Elizabeth Ann Seton, American saint (b. 1774)
- January 5 – Carlo Porta, Milanese poet (b. 1775)
- January 19 – Alexandru Suţu, prince of Moldavia (b. 1758)
- February 23 – John Keats, British poet (b. 1795)
- March 4 – Princess Elizabeth of Clarence, daughter of William, Duke of Clarence (later King William IV) (b.1820)
- March 13 – John Hunter, second Governor of New South Wales (b. 1737)
- April 10 – Patriarch Gregory V of Constantinople (b. 1746)
- May 5 – Napoleon Bonaparte (b. 1769)
- May 19 – Camille Jordan, French politician (b. 1771)
- June 7 – Tudor Vladimirescu, Wallachian rebellion-leader (b. c. 1780)
- June 17 – Martín Miguel de Güemes, Argentine military leader (b. 1785)
- June 30 – José Fernando de Abascal y Sousa, viceroy of Peru (b. 1743)
- July 4 – Richard Cosway, English artist (b. 1742)
- August 24 – John Polidori, physician and writer (b. 1795) (suicide)
- September 4 – José Miguel Carrera, Chilean general and founding father (b. 1785)
- September 10 – Johann Dominicus Fiorillo, German painter and art historian
- October 8 – Juan O'Donojú, viceroy of New Spain (b. 1762)
- October 11 – John Ross Key, commissioned officer in the Continental Army, judge, lawyer and the father of Francis Scott Key (b. 1754)
- October 21 – Dorothea Ackermann, German actress (b. 1752)
- November 8 – Jean Rapp, French general (b. 1771)
- December 7 – Pōmare II, king of Tahiti (b. 1782)
- December 12 – Phoebe Hessel, female soldier (b. 1713)