|Centuries:||18th century – 19th century – 20th century|
|Decades:||1790s 1800s 1810s – 1820s – 1830s 1840s 1850s|
|Years:||1818 1819 1820 – 1821 – 1822 1823 1824|
|1821 in topic:|
|Archaeology – Architecture – Art – Literature – Music|
|Australia – Canada – France – Germany – Mexico – Philippines – South Africa – US – UK|
|Rail Transport – Science – Sports|
|Lists of leaders|
|Colonial Governors – State leaders|
|Birth and death categories|
|Births – Deaths|
|Establishments and disestablishments categories|
|Establishments – Disestablishments|
|Ab urbe condita||2574|
|British Regnal year||1 Geo. 4 – 2 Geo. 4|
— to —辛巳年十二月初八日
|- Vikram Samvat||1877–1878|
|- Shaka Samvat||1743–1744|
|- Kali Yuga||4922–4923|
|- Ǹrí Ìgbò||821–822|
|Japanese calendar||Bunsei 4
|Juche calendar||N/A (before 1912)|
|Julian calendar||Gregorian minus 12 days|
|Minguo calendar||91 before ROC
|Thai solar calendar||2364|
|Wikimedia Commons has media related to: 1821|
- January 21 – Peter I Island in the Antarctic first sighted by Fabian Gottlieb von Bellingshausen.
- January 28 – Alexander Island, the largest in Antarctica, is first discovered by Fabian Gottlieb von Bellingshausen.
- February 9 – The George Washington University is chartered as The Columbian College of the District of Columbia by President Monroe.
- March 4 – James Monroe is sworn in for his second term as President of the United States.
- March 25 (O.S.)/April 6 (N.S.) – Metropolitan bishop Germanos of Patras raises the revolutionary flag of Greece at the Monastery of Agia Lavra, symbolically marking the beginning of the Greek War of Independence against the Ottoman Empire; later celebrated as Greece's traditional Independence Day (although more recent historical evidence suggests that the war had started before this date).
- April 10 – Ecumenical Patriarch Patriarch Gregory V of Constantinople is blamed by the Ottoman government for being unable to suppress Greek independence and is hanged outside the main gate of the Patriarchal Cathedral immediately after the celebration of Easter.
- May 5 – Emperor Napoléon I dies in exile on Saint Helena of stomach cancer.
- May 26 – Establishment of the Peloponnesian Senate by the Greek rebels.
- June 14 – King Badi VII of Sennar surrenders his throne and realm to Ismail Pasha, general of the Ottoman Empire, ending the existence of that Sudanese kingdom.
- June 19 – The Philikí Etaireía are decisively defeated by the Ottomans at Drăgăşani (in Wallachia).
- June 24 – Battle of Carabobo: Simón Bolívar wins Venezuela's independence from Spain.
- June 27 – The New Hampton School is founded in the United States state of New Hampshire.
- 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.
- September 4 – Chilean general José Miguel Carrera is executed by an Argentinian military tribunal in the city of Mendoza.
- September 4 – The Ukase of 1821: Russia proclames territorial sovereignty over Northwestern North America, now present day Alaska.
- 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 23 – Greek War of Independence: Siege of Tripolitsa ends when Greek rebels capture the city of Tripoli, Greece.
- September 15 – Guatemala, El Salvador, Honduras, Nicaragua and Costa Rica gain independence from Spain.
- September 18 – Amherst College is founded in Massachusetts.
- September 27 – Mexico gains its independence from Spain following the Mexican War of Independence.
- November 16 – American Old West: The Santa Fe Trail is first used by William Becknell.
- November 28 – Panama declares independence from Spain, joining Gran Colombia (see History of Panama).
- December 1 – The Dominican Republic declares independence from Spain (see History of the Dominican Republic). It will be invaded by Haiti in 1822.
- December 6 – The South Orkney Islands are discovered by George Powell and Nathaniel Palmer.
- December 19 – The Eyjafjallajökull volcano in Iceland begins to erupt.
Date unknown 
- January 8
- January 16 – John C. Breckinridge, United States
Vice president and Confederate Secretary of State.
- February 3 – Elizabeth Blackwell, first American female physician (d. 1910).
- February 11
royal mistress (d. 1861)
- February 19 – August Schleicher, German linguist (d. 1868)
- March 1 – Joseph Hubert Reinkens, German [[Old
Catholic Church|Old Catholic]] bishop (d. 1896)
philanthropist, and soldier (d. 1907)
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.
golfer (d. 1908)
politician, president of the Country (d. 1906)
- July 1 – Anatole Jean-Baptiste Antoine de Barthélemy, French archaeologist (d. 1904)
- July 9 – George Cavendish-Bentinck, British Conservative politician (d. 1891)
- 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)
- 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 Dostoevsky, 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 Garcia 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 I of France (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)
- 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 – Pomare II, king of Tahiti (b. 1782)
- December 12 – Phoebe Hessel, female soldier (b. 1713)