1866 (MDCCCLXVI) 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 1866th year of the Common Era (CE) and Anno Domini (AD) designations, the 866th year of the 2nd millennium, the 66th year of the 19th century, and the seventh year of the 1860s decade. As of the start of 1866, the Gregorian calendar was 12 days ahead of the Julian calendar, which remained in localised use until 1918.
- January 1
- January 6 – Ottoman troops clash with men of a Maronite leader Youssef Bey Karam at St. Doumit in Lebanon; the Ottomans are defeated.
- January 12
- January 18 – Wesley College, Melbourne is established.
- February 7 – Battle of Abtao: A Spanish naval squadron fights a combined Peruvian-Chilean fleet, at the island of Abtao in the Chiloé Archipelago of central Chile.
- February 13 – The first daylight bank robbery in United States history throughout peacetime takes place in Liberty, Missouri. This is considered to be the first robbery committed by Jesse James and his gang, although James's role is disputed.
- February 26 – The Calaveras Skull is discovered in California. Purported to be evidence of humans in North America throughout the Pliocene epoch, it turns out to be a hoax.
- February 28 – The month concludes without having a full moon.
- March 13 – The United States Congress overwhelmingly passes the Civil Rights Act of 1866, the first federal legislation to protect the rights of African-Americans; U.S. President Andrew Johnson vetoes the bill on March 27, and Congress overrides the veto on April 9.
- April 4 – Alexander II of Russia narrowly escapes an assassination attempt in the city of St Petersburg.
- April 8 – The kingdoms of Italy and Prussia form an alliance against the Austrian Empire.
- April 10 – The American Society for the Prevention of Cruelty to Animals (ASPCA) is founded in New York City by Henry Bergh.
- May 2 – Battle of Callao: Peruvian defenders fight the Spanish fleet.
- May 7 – Student Ferdinand Cohen-Blind makes a failed attempt to assassinate Otto von Bismarck in Unter den Linden in Berlin.
- May 10 – London bank Overend, Gurney and Company collapses, precipitating Panic of 1866.
- May 16 – The United States Congress approves the minting of a nickel 5-cent coin (nickel), eliminating its predecessor, the half dime.
- May 24 – Battle of Tuyutí: 32,000 soldiers of the Triple Alliance defeat 24,000 Paraguayan soldiers few miles north of the Paraná, Argentina in the Paraguayan War, with 16,000 casualties.
- May 26 – First production of the comic opera Cox and Box by F. C. Burnand and Arthur Sullivan at Moray Lodge, Kensington
- June 2 – Fenian forces skirmish with Canadian militia at the battles of Ridgeway and Fort Erie.
- June 5 – Calculations indicate Pluto (not known at this time) reaches its only aphelion (furthest point from the Sun) between 1618 and August 2113.
- June 8 – The Canadian Parliament meets for the first time in Ottawa.
- June 11 – The Agra High Court is established (later shifted to the Allahabad High Court).
- June 14 – The Austro-Prussian War begins, when the Austrians and most of the medium German states declare war on Prussia.
- June 20 – The Kingdom of Italy declares war on Austria.
- June 22 – In Sweden, the Riksdag of the Estates votes to replace itself by an elected 2-chamber Riksdag.
- June 27–June 29 – Battle of Langensalza: The Prussians defeat the Hanoverian army.
- July 1 – The first Constitution of Romania is issued.
- July 3 – Battle of Königgrätz: the Prussian army under King Wilhelm and Helmuth von Moltke defeats the Austrian army of Ludwig von Benedek, leading to a decisive Prussian victory in the Austro-Prussian War.
- July 5 – Princess Helena, third daughter of Queen Victoria, marries Prince Christian of Schleswig-Holstein.
- July 20 – Naval Battle of Lissa: The Austrian fleet under Wilhelm von Tegetthoff defeats the Italian fleet of Carlo di Persano.
- July 24 – Reconstruction: Tennessee becomes the first U.S. state to be readmitted to the Union following the American Civil War.
- July 25 – The United States Congress passes legislation authorising the rank of General of the Army (now called "5-star general"); Lieutenant General Ulysses S. Grant becomes the first to have this rank.
- July 27 – The SS Great Eastern successfully completes laying the transatlantic telegraph cable between Valentia Island, Ireland and Heart's Content, Newfoundland, permanently restoring a communications link.
- July 28 – The Metric Act of 1866 becomes law and legalises the standardisation of weights and measures in the United States.
- August 23 – The Treaty of Prague ends the Austro-Prussian War. The Duchy of Limburg leaves the German Confederation.
- September – The Great Tea Race of 1866 ends in London, narrowly won by the clipper ship Taeping.
- September 22 – Paraguay successfully defends Curupayty against the Triple Alliance in the Paraguayan War, killing more than 5,000 with just about 50 casualties.
- October 12 – The Treaty of Vienna ends the war between Austria and Italy; it formalises the annexation of Venetia by Italy.
- October 14 – French troops under the command of Rear Admiral Pierre-Gustave Roze land at Ganghwa Island, Korea as part of a punitive expedition against that kingdom for the execution of French Jesuit priests. It is the first military contact between Korea and a Western force.
- November 7 – The Ruse–Varna railway line (the first railway in Bulgaria) officially opens.
- December 18 – The College of Wooster is founded in Ohio.
- Federalist revolts occur in Argentina.
- Alfred Nobel invents dynamite in Germany.
- Foundation of the predecessors of Nestlé S.A., the Anglo-Swiss Milk Company and Farine Lactée Henri Nestlé.
- The Minneapolis Milling Company, predecessor of General Mills, builds its own mills.
- Marcus Jastrow arrives in the United States to become rabbi of Congregation Rodeph Shalom in Philadelphia.
- The recommendations of the state Girl School Committee of 1866 result in a series of progressive reforms in women's rights in Sweden.
- January 5 – William B. Hanna, American sportswriter (d. 1930)
- January 13
- January 15
- January 16 – Percy Pilcher, English inventor and pioneer aviator (d. 1899)
- January 29 – Romain Rolland, French writer, Nobel Prize laureate (d. 1944)
- February 2 – Enrique Simonet, Spanish painter (d. 1927)
- February 9 – George Ade, American writer, newspaper columnist, and playwright (d. 1944)
- February 18 – Janko Vukotić, Montenegrin general (d. 1927)
- February 19 – Louis-Henri Foreau, French painter (d. 1938)
- February 26 – Herbert Henry Dow, Canadian chemical industrialist (d. 1930)
- March 5 – Arthur Leopold Busch, English-born American submarine pioneer (d. 1956)
- March 7 – Hans Fruhstorfer, German lepidopterist (d. 1922)
- March 19 – Emilio de Bono, Italian general and fascist activist (d. 1944)
- March 30 – George Van Haltren, American baseball player (d. 1945)
- April 1
- April 13 – Butch Cassidy, American outlaw (k. 1909)
- April 14 – Anne Sullivan, American tutor of Helen Keller (d. 1936)
- April 17 – Ernest Starling, English physiologist (d. 1927)
- April 22 – Hans von Seeckt, German general (d. 1936)
- April 24 – Ishii Kikujirō, Japanese diplomat (d. 1945)
- May 5 – Thomas B. Thrige, Danish industrialist (d. 1938)
- May 17 – Erik Satie, French composer (d. 1925)
- May 22 – Charles F. Haanel, American New Thought author and businessman (d. 1949)
- June 4 – Miina Sillanpää, Finnish politician (d. 1952)
- June 26 – George Herbert, fifth Earl of Carnarvon, English financier of Egyptian excavations (d. 1923)
- June 29 – Bartholomeus Roodenburch, Dutch swimmer (d. 1939)
- July 3 – Albert Gottschalk, Danish painter (d. 1906)
- July 6 – Charles Mangin, French general (d. 1925)
- July 13 – La Goulue, French dancer (d. 1929)
- July 14 – Juliette Wytsman, Belgian painter (d. 1925)
- July 28 – Frederick Blackman, English plant physiologist (d. 1947)
- July 28 – Beatrix Potter, English children's author (Peter Rabbit & Jemima Puddle-Duck) (d. 1943)
- August 2 – Adrien de Gerlache, Belgian naval officer and explorer (d. 1934)
- August 5 – Carl Harries, German chemist (d. 1923)
- August 8 – Matthew Henson, African-American explorer (d. 1955)
- August 12 – Jacinto Benavente, Spanish writer, Nobel Prize laureate (d. 1954)
- August 14 – Dmitry Merezhkovsky, Russian novelist, poet, religious thinker (d. 1941)
- August 17 – Julia Marlowe, English-born American stage actress (d. 1950)
- September 1 – James J. Corbett, American boxer (d. 1933)
- September 7 – Tristan Bernard, French writer (d. 1947)
- September 10 – Jeppe Aakjær, Danish poet and novelist (d. 1930)
- September 16 – Joe Vila, American sportswriter (d. 1934)
- September 21
- September 22 – Witmer Stone, American ornithologist and botanist (d. 1939)
- September 27 – Eurosia Fabris, Italian Catholic Blessed (d. 1932)
- September 25 – Thomas Hunt Morgan, American geneticist, recipient of the Nobel Prize in Physiology or Medicine (d. 1945)
- October 6 – Reginald Fessenden, Canadian inventor (d. 1932)
- October 12 – Ramsay MacDonald, Scottish Prime Minister of the United Kingdom (d. 1937)
- October 26 – Dmitri Parsky, Russian general (d. 1921)
- November 12 – Sun Yat-sen, Chinese revolutionary (d. 1925)
- November 27 – George H. Reed, African-American screen actor (d. 1952)
- November 28
- November 30 – Robert Broom, Scottish palaeontologist (d. 1951)
- December 12 – Alfred Werner, German chemist, Nobel Prize laureate (d. 1919)
- December 16 (December 4 O.S.) – Wassily Kandinsky, Russian-born painter (d. 1944)
- January (date unknown) – Thomas Baldwin Marsh, American religious leader (b. 1799)
- January 16 – Phineas Quimby, American physician (b. 1802)
- January 19 – Harriet Ludlow Clarke, British artist
- January 23 – Thomas Love Peacock, English satirist (b. 1785)
- January 31 – Friedrich Rückert, German poet, translator and professor of Oriental languages (b. 1788)
- February 25 – Sarah Ann Gill, Barbadian national heroine (b. 1795)
- March 4 – Alexander Campbell, Irish/U.S. founder of the Disciples of Christ (b. 1788)
- March 6 – William Whewell, English scientist, philosopher and historian of science (b. 1794)
- March 20 – Rikard Nordraak, Norwegian composer (b. 1842)
- March 21 – Nadezhda Durova, first female Russian military officer (b. 1783)
- March 28 – Solomon Foot, American politician (b. 1802)
- March 29 – John Keble, British churchman (b. 1792)
- April 1 – Elizabeth Jesser Reid, English social reformer, founder of Bedford College (b. 1789)
- April 4 – William Dick, founder of Edinburgh Veterinary College (b. 1793)
- April 5 – Thomas Hodgkin, British physician (b. 1798)
- April 12 – Peter Hesketh-Fleetwood, English Member of Parliament and developer (b. 1801)
- May 13 – Nikolai Brashman, Russian mathematician of Czech origin (b.1796)
- May 29 – Winfield Scott, American general and presidential candidate (b. 1786)
- June 7 – Chief Sealth, Native American for whom Seattle is named (b. c. 1786)
- July 20 – Bernhard Riemann, German mathematician (b. 1826)
- July 25 – Floride Calhoun, Second Lady of the United States (b. 1792)
- August 6 – Christian Eric Fahlcrantz, Swedish writer (b. 1790)
- August 20 – Maria De Mattias, Catholic saint (b. 1805)
- August 29 – Tokugawa Iemochi, fourteenth shogun of the Tokugawa shogunate of Japan (b. 1846)
- October 13 – Celadon Leeds Daboll, American merchant and inventor (b. 1818)
- November 11 – Agustín Jerónimo de Iturbide y Huarte, Prince Imperial of Mexico (b. 1807)
- November 14 – King Miguel I of Portugal (b. 1802)
- November 26 – Jean-Jacques Willmar, Luxembourg politician (b. 1792)
- December 1 – George Everest, Welsh geodesist (b. 1790)
- September 21 – Mercedes Marín del Solar, international Chilean poet and reform educator (b. 1804)