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|Centuries:||18th century – 19th century – 20th century|
|Decades:||1790s 1800s 1810s – 1820s – 1830s 1840s 1850s|
|Years:||1817 1818 1819 – 1820 – 1821 1822 1823|
|1820 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||2573|
|British Regnal year||60 Geo. 3 – 1 Geo. 4|
— to —庚辰年十一月廿六日
|- Vikram Samvat||1876–1877|
|- Shaka Samvat||1742–1743|
|- Kali Yuga||4921–4922|
|- Ǹrí Ìgbò||820–821|
|Japanese calendar||Bunsei 3
|Juche calendar||N/A (before 1912)|
|Julian calendar||Gregorian minus 12 days|
|Minguo calendar||92 before ROC
|Thai solar calendar||2363|
|Wikimedia Commons has media related to: 1820|
- January 1 – Trienio Liberal in Spain: A constitutionalist military insurrection at Cádiz leads to the summoning of the Spanish Parliament (March 7).
- January 20 – Indiana University Bloomington is founded as the Indiana State Seminary (renamed Indiana College, 1846).
- January 28 (NS) – History of Antarctica: An Imperial Russian Navy expedition led by Fabian Gottlieb von Bellingshausen and Mikhail Petrovich Lazarev sights the coast of Antarctica.
- January 29 – George IV of the United Kingdom ascends the throne on the death at Windsor Castle of his father George III (after 59 years on the throne), ending the period known as the British Regency. There will be a gap of 21 years before the title Prince of Wales is next used.
- January 30 – History of Antarctica: British Royal Navy captain Edward Bransfield lands on the mainland of Antarctica.
- February 6
- February 14 – Minh Mang starts to rule in Vietnam.
- February 20 – A revolt begins in Santa María Chiquimula, Totonicapán department of Guatemala.
- February 20 The Daoguang Emperor succeeds the throne of the Qing Dynasty in China.
- February 23 – A plot to murder the Cabinet of the United Kingdom, the Cato Street conspiracy, is exposed in England; the principals are the last to suffer decapitation following their hanging for treason outside Newgate Prison in London on May 1.
- March 3 and March 6 – Slavery in the United States: The Missouri Compromise becomes law, allowing admission of Missouri and Maine, slave and free states respectively, as U.S. states.
- March 9 – King Ferdinand VII of Spain accepts the new constitution, beginning the Trienio Liberal.
- March 15 – Maine is admitted as the 23rd U.S. state.
- April – Hans Christian Ørsted discovers the relationship between electricity and magnetism.
- April 1 – A Proclamation, signed "By order of the Committee of Organisation for forming a Provisional Government", begins the "Radical War" in Scotland.
- April 8 – The statue of the Venus de Milo (Aphrodite of Milos, c.150 BC-125 BC) is discovered on the Greek island of Milos by a peasant named Yorgos Kentrotas.
- April 12 – Alexander Ypsilantis is declared leader of Filiki Eteria, a secret organization to overthrow Ottoman rule over Greece.
- April 15 – King William I of Württemberg marries his cousin, Pauline Therese, in Stuttgart.
- May 1 – The last hanging, drawing and quartering in Britain is meted out to the Cato Street conspirators for treason (only hanged and beheaded).
- May 11 – HMS Beagle (the ship that will later take young Charles Darwin on his scientific voyage) is launched at Woolwich Dockyard.
- May 20 – John Stuart Mill sets out on his formative boyhood trip to France.
- July – A revolt under Guglielmo Pepe forces Ferdinand I of the Two Sicilies to sign a constitution modeled on the Spanish Constitution of 1812.
- July 20 – Saint Cronan's Boys National School opens in Bray, Co. Wicklow, Ireland under the title Bray Male School. It is the oldest school in Bray and its notable pupils will include President of Ireland Cearbhall Ó Dálaigh.
- July 26 – Opening of Union Chain Bridge across the River Tweed between England and Scotland, a wrought iron suspension bridge designed by Captain Samuel Brown. Its span of 449 ft (137 m) is the world's longest for a vehicular bridge at this time.
- August 24 – A Constitutionalist insurrection breaks out at Oporto, Portugal.
- September 15 – Revolution breaks out in Lisbon against John VI of Portugal.
- October 9 – History of Ecuador: Guayaquil declares independence from Spain.
- October 25–November 20 – The Congress of Troppau (Opava) is convened between the rulers of Russia, Austria and Prussia.
- November 20 – After the sinking of the whaleship Essex of Nantucket by a whale the survivors are left afloat in three small whaleboats. They eventually resort, by common consent, to cannibalism to allow some to survive.
- December 3 – U.S. presidential election: James Monroe is re-elected, virtually unopposed.
- December 20 – The town of Tuscumbia, Alabama, is incorporated.
Date unknown 
- Early Spring – Joseph Smith, Jr. receives his First Vision in Palmyra, New York.
- The Argentine Confederation (Argentina) establishes a penal colony in the Malvinas.
- Robert Owen devises the labour voucher.
- Mount Rainier erupts over what is today Seattle.
- 18,957 black slaves leave Luanda, Angola.
- The 6th edition of the Encyclopædia Britannica appears.
- Construction work is completed on the Citadelle Laferrière in Haiti, the largest fortification in the Americas.
- Anchor coinage is issued for use in some British colonies.
- January 10 – Louisa Lane Drew, actress and prominent theater manager, grandmother of the Barrymores (d. 1897)
- January 17 – Anne Brontë, English author (d. 1849)
- February 1 – George Hendric Houghton, American Protestant Episcopal clergyman (d. 1897)
- February 8 – William Tecumseh Sherman, American Civil War general (d. 1891)
- February 13 – James Geiss, English businessman (d. 1878)
- February 15 – Susan B. Anthony, American suffragist (d. 1906)
- February 17 – Henri Vieuxtemps, Belgian violinist and composer (d. 1881)
- February 28 – John Tenniel, English illustrator (d. 1914)
- March 2 – Eduard Douwes Dekker, Dutch writer (d. 1887)
- March 3 – Henry D. Cogswell, American temperance movement pioneer who endowed a number of Cogswell fountains (d. 1900)
- March 4 – Francesco Bentivegna, Italian revolutionary (d. 1856)
- March 20 – Alexandru Ioan Cuza, Romania's first reigning Domnitor (d. 1873)
- March 14 – Victor Emmanuel II of Italy (d. 1878)
- April 27 – Herbert Spencer, English philosopher (d. 1903)
- April 26 – Alice Cary, American poet, sister to Phoebe Cary (1824-1871) (d. 1871)
- May 12 – Florence Nightingale, English nurse (d. 1910)
- May 23 – Lorenzo Sawyer, 9th Chief Justice of the Supreme Court of California (d. 1891)
- May 27 – Mathilde Bonaparte, Italian princess (d. 1904)
- July 9 – John Wright Oakes, landscape painter (d. 1887)
- July 23 – Julia Gardiner Tyler, First Lady of the United States (d. 1889)
- September 17 – Émile Augier, French dramatist (d. 1889)
- September 20 – John F. Reynolds, American general (d. 1863)
- September 27 – Wilhelm Siegmund Teuffel, German classical scholar (d. 1878)
- September 29 – Henry, Count of Chambord, claimant to the French throne (d. 1883)
- October 5 – David Wilber, American politician (d. 1890)
- October 6 – Jenny Lind, Swedish soprano (d. 1887)
- November 23 – Isaac Todhunter, English mathematician (d. 1884)
- November 28 – Friedrich Engels, German social philosopher (d. 1895)
- January 17 – Daniel Albert Wyttenbach, academic (b. 1746)
- January 29 – King George III of the United Kingdom (b. 1738)
- February 11 – Karl von Fischer, architect (b. 1782)
- February 14 – Charles Ferdinand, Duke of Berry (stabbed) (b. 1778)
- March 22 – Stephen Decatur, American sailor (b. 1779)
- May 30 – William Bradley, Britain's tallest ever man (b. 1787)
- June 9 – Wilhelmina of Prussia, Princess of Orange (b. 1751)
- June 19 – Sir Joseph Banks, British naturalist and botanist (b. 1743)
- June 20 – Manuel Belgrano, Argentine politician and general in the Independence War (b. 1770)
- August 12 – Manuel Lisa, fur trader (b. 1772)
- September 3 – Benjamin Latrobe, English architect (b. 1764)
- September 4 – Timothy Brown, English banker, merchant and radical (b. 1743/1744)
- September 26 – Daniel Boone, American pioneer (b. 1734)
- September 29 – Barthelemy Lafon, architect and smuggler (b. 1769)
- October 15 – Karl Philipp Fürst zu Schwarzenberg, Austrian field marshal (b. 1771)
- December 25 – Joseph Fouché, French statesman (b. 1763)
Date unknown 
- Jones, A. G. E. (1982). Antarctica Observed: who discovered the Antarctic Continent?. Caedmon of Whitby. ISBN 0-905355-25-3.
- Drewry, Charles Stewart (1832). "Section III". A Memoir of Suspension Bridges: Comprising The History Of Their Origin And Progress. London: Longman, Rees, Orme, Brown, Green & Longman. pp. 37–41. Retrieved 2011-08-16.