1823 (MDCCCXXIII) was a common year starting on Wednesday (dominical letter E) of the Gregorian calendar and a common year starting on Monday (dominical letter G) of the Julian calendar, the 1823rd year of the Common Era (CE) and Anno Domini (AD) designations, the 823rd year of the 2nd millennium, the twenty-third year of the 19th century, and the fourth year of the 1820s decade. Note that the Julian day for 1823 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.
- January 23 – In Paviland Cave on the Gower Peninsula of Wales, William Buckland inspects the "Red Lady of Paviland", the first identification of a prehistoric (male) human burial.
- February 3 – Gioachino Rossini's Semiramide is first performed.
- February 10 – First worldwide carnival parade took place in Cologne, Prussia.
- February 20 – Explorer James Weddell's expedition to Antarctica reaches latitude 74°15' S and longitude 34°16'45" W: the southernmost position any ship had reached before, a record that will hold for more than 80 years.
- March 19 – Agustín de Iturbide, Emperor of Mexico, abdicates thus ending the short-lived First Mexican Empire.
- April 13 – Eleven-year-old Franz Liszt gives a concert after which he's personally congratulated by Ludwig van Beethoven.
- June 5 – Raffles Institution established as the Singapore Institution by the founder of Singapore, Sir Stamford Raffles.
- July 1 – The congress of Central America declares absolute independence from Spain, Mexico, and any additional foreign nation, including North America, and a republican system of government is established.
- July – Robert Peel ensures the passage of five Acts of Parliament in the United Kingdom, effectively abolishing the death penalty for over one hundred offences; in particular, the Judgement of Death Act allows judges to commute sentences for capital offences additional than murder or treason to imprisonment or transportation.
- July 10 – Gaols Act passed by Parliament of the United Kingdom, based on the prison reform campaign of Elizabeth Fry.
- July 15 – The Basilica of Saint Paul Outside the Walls in Rome is almost completely destroyed by fire.
- September 10 – Simón Bolívar is named President of Peru.
- September 22 – Joseph Smith claimed in 1838 that on this day he had first come to the place where the golden plates were stored, having been directed there by God through an angel.
- September 23 – First Anglo-Burmese War: Burmese attack the British on Shapura, an island close to Chittagong.
- October 5 – Medical journal The Lancet is founded by Thomas Wakley in London.
- November – According to tradition, William Webb Ellis invents the sport of rugby football at Rugby School in England.
- December 2 – James Monroe first introduces the Monroe Doctrine in the State of the Union address, declaring that any European attempts to recolonize the Americas would be considered a hostile act towards the United States.
- Beginning of the first Anglo-Ashanti war.
- Olbers' paradox is described by the German astronomer Heinrich Wilhelm Olbers.
- Work begins on the British Museum in London, designed by Robert Smirke, and the Altes Museum in Berlin, designed by Karl Friedrich Schinkel.
- Jackson Male Academy, precursor of Union University, is founded in Tennessee.
- The Oxford Union is founded.
- January 1 – Sándor Petőfi, Hungarian poet and revolutionary (d. 1849)
- January 8 – Alfred Russel Wallace, British naturalist and biologist (d. 1913)
- January 27 – Édouard Lalo, French composer (d. 1892)
- February 15 – Li Hongzhang, Chinese politician, general, and diplomat (d. 1901)
- February 27 – Ernest Renan, French philosopher and writer (d. 1892)
- February 28 – Frederick Francis II, Grand Duke of Mecklenburg-Schwerin (d. 1883)
- March 14 – Théodore de Banville, French writer (d. 1891)
- March 20 – Ned Buntline, American publisher, writer, and publicist (d. 1886)
- March 23 – Schuyler Colfax, Vice President of the United States (d. 1885)
- April 1 – Simon Bolivar Buckner, American soldier and politician and Confederate soldier (d. 1914)
- April 3 – William M. Tweed, American political boss (d. 1878)
- April 4 – Carl Wilhelm Siemens, German engineer (d. 1883)
- April 23 – Abdülmecid I, Ottoman Sultan (d. 1861)
- May 2 – Emma Hardinge Britten, born Emma Floyd, English-born spiritualist (d. 1899)
- May 15
- May 17 – Henry Eckford, British horticulturist (d. 1905)
- May 22 – Solomon Bundy, American politician (d. 1889)
- May 26 – William Pryor Letchworth, American businessman and philanthropist, founder of Letchworth State Park
- July 6 – Sophie Adlersparre, Swedish feminist (d. 1895)
- June 13 – David Breakenridge Read, Canadian lawyer, Mayor of Toronto (d. 1904)
- June 21 – Jean Chacornac, French astronomer (d. 1873)
- July 18
- July 23 – Coventry Patmore, English poet (d. 1896)
- August 3 – Thomas Francis Meagher, American Civil War general (d. 1867)
- August 4 – Oliver P. Morton, American politician (d. 1877)
- August 5 – Eliza Tibbets, mom of the California orange industry (d. 1898)
- August 10
- August 11– Charlotte Mary Yonge, English Author (d. 1901)
- August 13 – Goldwin Smith, English historian (d. 1910)
- August 14 – Karel Miry, Belgian composer (d. 1889)
- August 15 – Orris S. Ferry, American Civil War general and politician (d. 1875)
- August 26 – Wilhelm Troszel, Polish composer (d. 1887)
- September 15 – Hugh Buchanan, American politician (d. 1890)
- September 23 – James Black, American temperance movement leader (d. 1893)
- November 8 – Joseph Monier, French inventor (d. 1906)
- November 18 – Charles H. Bell American politician (d. 1893)
- November 25 – Henry Wirz, Confederate military officer and prisoner-of-war camp commander (d. 1865)
- December 6 – Friedrich Max Müller, German Orientalist (d. 1900)
- December 13 – Ferdinand Büchner, German composer (d. 1906)
- December 22 – Thomas Wentworth Higginson, American Unitarian minister and abolitionist (d. 1911)
- December 27 – Mackenzie Bowell, fifth Prime Minister of Canada (d. 1917)
- January 21 – Gideon Olin, American politician (b. 1743)
- January 26 – Edward Jenner, English physician and medical researcher (b. 1749)
- February 7 – Ann Radcliffe, English writer (b. 1764)
- February 21 – Charles Wolfe, Irish poet (b. 1791)
- March 1 – Pierre-Jean Garat, French Basque opera singer (b. 1764)
- March 14
- March 18 – Jean-Baptiste Bréval, French cellist (b. 1753)
- June 1 – Louis-Nicolas Davout, French marshal (b. 1770)
- June 19 – William Combe, English writer, poet and adventurer (b. 1742)
- August 7 – Mátyás Laáb, Croatian writer and translator (b. 1746)
- August 20 – Pope Pius VII, Italian Benedictine (b. 1742)
- August 22 – Lazare Carnot, French general, politician and mathematician (b. 1753)
- September 11 – David Ricardo, English economist (b. 1772)
- September 23 – Matthew Baillie, Scottish physician and pathologist (b. 1761)
- September 28 – Charlotte Melmoth, English-born American actress (b. 1749)
- November 9 – Vasily Kapnist, Ukrainian Russian poet and dramatist (b. 1758)
- December 3 – Giovanni Battista Belzoni, Italian explorer and pioneer archaeologist of Egypt (b. 1778)