1866 is a common year starting on Monday.
1863 1864 1865 - 1866 - 1867 1868 1869
1830s 1840s 1850s - 1860s - 1870s 1880s 1890s
18th century - 19th century - 20th century
- January 6 – Ottoman troops clash with men of a Maronite leader Karam in St. Doumit in Lebanon - Turks are defeated
- January 12 - Royal Aeronautical Society is formed (London)
- January 28 - 800 Maronite troops clash with Ottoman troops in Karem Saddah, modern-day Lebanon - more battles between nationalist Maronites and Ottoman army follow
- February 13 - Jesse James robs his first bank
- May - Student Choen Blind fails to assassinate Otto von Bismarck in Unter den Linden in Berlin
- May 16 - The United States Congress eliminates the half dime coin and replaces it with the five cent piece, or nickel.
- May 16 - Charles Elmer Hires invents root beer.
- May 24 - Battle of Tuyuti - 32.000 soldiers of the Triple Alliance defeat 24.000 Paraguayan soldiers few miles north of the Parana - 18.000 dead
- June 14 - Beginning of the Austro-Prussian War, when the Austrians and most of the medium German states declare war on Prussia.
- June 8 - The Canadian Parliament meets for the first time in Ottawa.
- July 3 - Austro-Prussian War decided at the [[Battle of K�niggratz]], resulting in Prussia taking over as the prominent German nation from Austria.
- July 5 - Marriage of Princess Helena of the United Kingdom, third daughter of Queen Victoria to Prince Christian of Schleswig-Holstein-Sonderburg-Augustenburg
- July 20 - Naval battle at Lissa between Austria Hungary and Italy.
- 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 authorizing 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 Atlantic Cable is successfully completed, allowing transatlantic telegraph communication for the first time.
- July 28 - The Metric Act of 1866 becomes law and legalizes the standardization of weights and measures in the United States.
- August 23 - Treaty of Prague ends the Austro-Prussian War
- September 22 - Paraguay sucessfully defends Curupaity against the Triple Alliance, scoring more than 5000 with just about 50 casualties.
- Federalist revolts in Argentina
- In Sweden the Riksdag of the Estates is replaced by an elected two chamber assembly, the Riksdag.
- Alfred Nobel invents dynamite.
- First historical mention of gerbils, "yellow rats" sent to Museum of Natural History (Mus�e d'Histoire Naturelle) in Paris, by father Armand David from northern China
- January 13 - Vasily Kalinnikov, composer
- January 29 - Romain Rolland, dramatist, winner of Nobel Prize in Literature in 1915 (+ 1944)
- April 1 - Ferruccio Busoni, pianist and composer (+ 1924)
- April 6 - Butch Cassidy, outlaw (+ 1909)
- May 17- Erik Satie, French composer
- July 28 - Beatrix Potter, Children's author
- September 1 - James J. Corbett, heavyweight boxing champion (d. 1933)
- September 7 - Tristan Bernard, writer
- September 21 - H. G. Wells, science fiction novelist
- Thomas Hunt Morgan, biologist
- Sun Yat-sen, Chinese Revolutionary
- Robert Broom, paleontologist
- La Goulue, Cancan dancer (d. 1929)
- January 31 - [[Friedrich R�ckert]], German poet, translator, and professor of Oriental languages (* 1788)
- November 26 - Jean-Jacques Willmar, luxembourgish politician (* 1792)
- Bernhard Riemann, Mathematician
- Rikard Nordraak, Norwegian composer
- January - Thomas Baldwin Marsh - early leader of the Church of Jesus Christ of Latter-Day Saints