1821 (MDCCCXXI) was a common year starting on Monday of the Gregorian calendar and a common year starting on Saturday 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. As of the start of 1821, the Gregorian calendar was 12 days ahead of the Julian calendar, which remained in localized use until 1923.
- January 21 – Peter I Island in the Antarctic is 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 – Columbian College of the District of Columbia is chartered by President Monroe.
- March 4 – James Monroe begins his second term as President of the United States.
- March 5 – President James Monroe is sworn in for his second term.
- 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.
- April 10 – Ecumenical 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 Napoleon dies in exile on Saint Helena of stomach cancer.
- May 26 – The Peloponnesian Senate is established 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 – Battle of Drăgășani, Wallachia: The Filiki Eteria are decisively defeated by the Ottomans.
- June 24 – Battle of Carabobo: Simón Bolívar wins Venezuela's independence from Spain.
- October 5 – The massacre of 8,000 civilians follows the end of the Siege of Tripolitsa (Greek forces under the command of General Theodoros Kolokotronis had besieged the city for several months, during the Greek War of Independence from Turkey and the Ottoman Empire.  
- October 8 – Tsar Alexander I of Russia issues an imperial ukase, guaranteeing freedom of commerce in Russia by merchants from Persia. 
- 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.
- December 1 – The Dominican Republic declares independence from Spain (see History of the Dominican Republic). It would be invaded by Haiti in 1822.
- December 6 – The South Orkney Islands are discovered, by George Powell and Nathaniel Palmer.
- December 15 – The world's first geographical society, the Société de géographie, is established in Paris.
- December 19 – The Eyjafjallajökull volcano in Iceland begins to erupt.
- January 2 – Catherine Huggins, British actor, singer, director and manager (d. 1887)
- January 8
- January 16 – John C. Breckinridge, the 14th Vice President of the United States (1857-1861) and Confederate Secretary of State in 1865 (d. 1875)
- February 3 – Elizabeth Blackwell, first American female physician (d. 1910)
- February 11
- February 17 – Lola Montez, Irish-Spanish dancer, royal mistress (d. 1861)
- February 19
- February 22 – Athalia Schwartz, Danish writer, journalist and educator (d. 1871)
- 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)
- March 31 – Henry Dunning Macleod, Scottish economist (d. 1902)
- April 1 – Princess Anka Obrenović, Serbian princess (d. 1868)
- April 3 – Fr. Thomas Pelham Dale English mystic (d. 1892)
- April 9 – Charles Baudelaire, French poet, writer (d. 1867)
- May 6
- 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 2 – Ion C. Brătianu, 2-Time Prime Minister of Romania (d. 1891)
- June 16 – Old Tom Morris, Scottish golfer (d. 1908)
- June 26 – Bartolomé Mitre, Argentine statesman, military figure, and author, 6th 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 13 – Nathan Bedford Forrest, American Confederate Civil War General, first Grand Wizard of the Ku Klux Klan (d. 1877)
- July 16 – Mary Baker Eddy, American founder of Christian Science (d. 1910)
- July 17 – Friedrich Engelhorn, German industrialist and founder of BASF (d. 1902)
- July 18 – Pauline Viardot, French mezzo-soprano, 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 16 – Arthur Cayley, English mathematician (d. 1895)
- 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, politician (d. 1874)
- October 13 – Rudolf Virchow, German physician, pathologist, biologist, and politician (d. 1902)
- October 20 – Eufrosina Popescu, Romanian actress (d. 1900)
- 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 22 – Junius Brutus Booth, Jr., American actor, theatre manager (d. 1883)
- 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)
- April 20– Franz Karl Achard, German chemist, physicist and biologist (b. 1753)
- April 23 – Pierre de Ruel, marquis de Beurnonville, French general (b. 1752)
- May 2 – Hester Thrale, Welsh diarist (b. 1741)
- May 5 – Emperor Napoleon Bonaparte (I) of France (b. 1769)
- May 19 – Camille Jordan, French politician (b. 1771)
- June 7 – Tudor Vladimirescu, Wallachian rebel leader (b. c. 1780)
- June 17 – Martín Miguel de Güemes, Argentine military leader (b. 1785)
- June 19 – Peter Ochs, Swiss politician (b. 1752)
- June 23 – Louise Marie Adélaïde de Bourbon-Penthièvre, Duchess of Orléans, heiress, wife of Philippe Égalité (b. 1753)
- June 30 – José Fernando de Abascal y Sousa, viceroy of Peru (b. 1743)
- July 4 – Richard Cosway, English artist (b. 1742)
- August – Dorothea von Medem, Latvian diploma, duchess of Courland (b. 1761)
- August 24 – John William Polidori, English physician, writer (b. 1795) (suicide)
- September 4 – José Miguel Carrera, Chilean general, founding father (b. 1785)
- September 10 – Johann Dominicus Fiorillo, German painter, art historian (b. 1748)
- September 14 – Heinrich Kuhl, German naturalist, zoologist (b. 1797)
- October 4 – Marie-Louise Lachapelle, French obstetrician (b. 1769)
- October 6 – Anders Jahan Retzius, Swedish chemist, botanist (b. 1742)
- October 8 – Juan O'Donojú, viceroy of New Spain (b. 1762)
- October 11 – John Ross Key, American judge, lawyer, father of songwriter Francis Scott Key (b. 1754)
- October 21 – Dorothea Ackermann, German actress (b. 1752)
- November 8 – Jean Rapp, French general (b. 1771)
- December 4 – John Henniker-Major, 2nd Baron Henniker, British politician (b. 1752)
- December 7 – King Pōmare II of Tahiti (b. 1782)
- December 12 – Phoebe Hessel, British female soldier (b. 1713)
- Benjamin Lieberman, Terrible Fate: Ethnic Cleansing in the Making of Modern Europe (Rowman & Littlefield, 2013) p9
- A Global Chronology of Conflict: From the Ancient World to the Modern Middle East, ed. by Spencer C. Tucker (ABC-CLIO, 2009) p1139
- "Commerce between Russia and Persia— Proposed Union of the Black Sea with the Atlantic", in The Oriental Herald (November 1826) p285
- Headland, Robert K. (1989). Chronological list of Antarctic expeditions and related historical events. Cambridge University Press. ISBN 978-0-521-30903-5. OCLC 185311468.
- "South Orkney Islands". Encyclopædia Britannica Online Academic Edition.
- "BBC - History - Historic Figures: John Keats (1795-1821)". bbc.co.uk. Retrieved 3 January 2017.
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