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1861 was a common year starting on Tuesday of the Gregorian calendar and a common year starting on Sunday of the Julian calendar, the 1861st year of the Common Era (CE) and Anno Domini (AD) designations, the 861st year of the 2nd millennium, the 61st year of the 19th century, and the 2nd year of the 1860s decade. As of the start of 1861, the Gregorian calendar was 12 days ahead of the Julian calendar, which remained in localized use until 1923.
- January 1
- January 2 – Friedrich Wilhelm IV of Prussia dies, and is succeeded by Wilhelm I.
- January 3 – American Civil War: Delaware votes not to secede from the Union.
- January 9 – American Civil War: Mississippi becomes the second state to secede from the Union.
- January 10 – American Civil War: Florida secedes from the Union.
- January 11 – American Civil War: Alabama secedes from the Union.
- January 12 – American Civil War: Major Robert Anderson sends dispatches to Washington.
- January 19 – American Civil War: Georgia secedes from the Union.
- January 21 – American Civil War: Jefferson Davis resigns from the United States Senate.
- January 26 – American Civil War: Louisiana secedes from the Union.
- January 29 – Kansas is admitted as the 34th U.S. state.
- February 1 – American Civil War: Texas secedes from the Union.
- February 4 – American Civil War: In Montgomery, Alabama, the Provisional Confederate States Congress is formed by representatives from the first seven break-away states.
- February 8 – American Civil War: The Confederate States of America are formed, comprising the first seven break-away States.
- February 9 – American Civil War: Jefferson Davis is elected the Provisional President of the Confederate States of America, by the Weed Convention at Montgomery, Alabama.
- February 11
- American Civil War: The U.S. House unanimously passes a resolution, guaranteeing non-interference with slavery in any state.
- About 850 convicts at Chatham Dockyard in England take over their prison in a riot.
- February 13 – Italian unification: The Siege of Gaeta, stronghold of the Neapolitan King Francis II, is ended by Piedmontese forces. Francis goes into exile.
- February 18 – American Civil War: In Montgomery, Alabama, Jefferson Davis is inaugurated as the provisional president of the Confederate States of America.
- February 20 – In Britain, storms damage the Crystal Palace and cause the collapse of the steeple of Chichester Cathedral.
- February 23 – President-elect Abraham Lincoln arrives secretly in Washington, D.C. after an assassination attempt in Baltimore.
- February 24 – Battle of Ky Hoa: the French and the Spanish defeat the Vietnamese.
- February 27 – Russian troops fire upon a crowd in Warsaw protesting Russian rule over Poland, killing 5 protesters.
- February 28 – Colorado is organized as a United States territory.
- March 2
- March 3 (February 19 O.S.) – Emancipation reform of 1861: Serfdom is abolished in the Russian Empire.
- March 4
- March 10 – El Hadj Umar Tall seizes the city of Ségou, destroying the Bamana Empire of Mali.
- March 11 – American Civil War: The Constitution of the Confederate States of America is adopted.
- March 13 – Tsushima incident: The Russian corvette Posadnik arrives at Tsushima Island in the Korea Strait, Japan, provoking a reaction from the Japanese Shogunate.
- March 17 – Italian unification: The Kingdom of Italy is proclaimed by the new Parliament, with Victor Emmanuel II of Piedmont-Sardinia becoming its king.
- March 19 – The First Taranaki War ends in New Zealand.
- March 20
- March 21 – Alexander Stephens, Vice President of the Confederacy, gives the infamous Cornerstone Speech in Savannah, Georgia, in which he declares that slavery is the natural condition of blacks, and the foundation of the Confederacy.
- March 28 – Confederate Arizona: convention in present-day Tucson ratified the ordinance of secession of southern part of New Mexico Territory.
- March 30 – Discovery of the chemical elements: Sir William Crookes announces his discovery of thallium.
- April 7 – A population census is taken in the United Kingdom.
- April 12 – The American Civil War begins with the bombardment of Fort Sumter, South Carolina.
- April 13 – American Civil War: Fort Sumter surrenders to Southern forces.
- April 15 – American Civil War: President Abraham Lincoln issues a Proclamation calling for 75,000 men to confront in the South, "combinations too powerful to be suppressed in the ordinary way".
- April 17 – American Civil War: The state of Virginia secedes from the Union.
- April 20 – American Civil War: Robert E. Lee resigns his commission in the United States Army, in order to command the forces of the state of Virginia.
- April 24 (N.S.) – Bezdna, Russia is the scene of a peasant uprising; the military open fire and nearly 5,000 are killed.
- April 25 – American Civil War: The Union Army arrives in Washington, D.C.
- April 26 – Giovanni Schiaparelli discovers the asteroid 69 Hesperia.
- April 27 – American Civil War:
- May 6 – American Civil War: Arkansas secedes from the Union.
- May 7 – American Civil War: Tennessee secedes from the Union.
- May 8 – American Civil War: Richmond, Virginia is named the capital of the Confederate States of America.
- May 13
- May 14 – The Canellas meteorite, an 859 gram chondrite type meteorite, strikes Earth near Barcelona, Spain.
- May 20 – American Civil War:
- May 21 – Russian sailors clash with a group of Japanese samurai and farmers, at Tsushima island.
- October 9 – American Civil War: Battle of Santa Rosa Island – Confederate forces are defeated in their effort to take the island.
- October 21 – American Civil War: Battle of Ball's Bluff – Union forces under Colonel Edward Baker are defeated by Confederate troops, in the second major battle of the war. Baker, a close friend of Abraham Lincoln, is also killed in the fighting.
- October 24 – HMS Warrior, the world's first ocean-going (all) iron-hulled armored battleship, is completed and commissioned into the British Royal Navy.
- October 25 – The Toronto Stock Exchange is established in Canada.
- October 26 – The Pony Express American transcontinental mail service announces its closure.
- October 28 – American Civil War: The Missouri legislature takes up a bill for Missouri's secession from the Union.
- October 30 – American Civil War: The bill for Missouri's secession from the Union is passed.
- October 31
- November 1 – American Civil War: U.S. President Abraham Lincoln appoints George B. McClellan as commander of the Union Army, replacing Winfield Scott.
- November 2 – American Civil War: Western Department Union General John C. Frémont is relieved of command and replaced by David Hunter.
- November 6 – American Civil War: Jefferson Davis is elected president of the Confederate States of America.
- November 5 – The first Melbourne Cup horse race is held in Melbourne, Australia.
- November 7 – American Civil War – Battle of Belmont: In Belmont, Missouri, Union forces led by General Ulysses S. Grant (in his first combat leadership role) overrun a Confederate camp, but are forced to retreat when Confederate reinforcements arrive.
- November 8 – American Civil War – Trent Affair: The USS San Jacinto stops the United Kingdom mail ship Trent, and arrests two Confederate envoys, James Mason and John Slidell, sparking a diplomatic crisis between the U.K. and U.S.
- November 10 – Following the death of Henri Mouhot, his servant Phrai begins shipping his diaries and specimens back to the west; they include accounts of Mouhot's discovery of Angkor Wat.
- November 19 – American Civil War: Battle of Round Mountain in Indian Territory (modern-day Oklahoma).
- November 21 – American Civil War: Confederate President Jefferson Davis appoints Judah Benjamin Secretary of War.
- November 25 – A tenement collapses in the Old Town, Edinburgh (Scotland), killing 35 with 15 survivors.
- November 28 – Acting on the ordinance passed by the Jackson government, the Confederate Congress admits Missouri as the 12th Confederate state.
- December 10
- December 21 - Medal of Honor: Public Resolution 82, containing a provision for a Navy Medal of Valor, is signed into law by President Abraham Lincoln.
- January 6 – János Zsupánek, Slovene (Prekmurje Slovene) poet and writer (d. 1951)
- January 10 – Metropolitan Germogen (d. 1945)
- January 14 – Mehmed VI, Ottoman Sultan (d. 1926)
- January 28 – Julián Felipe, Filipino musician, bandleader (d. 1944)
- January 30 – Charles Martin Loeffler, American composer (d. 1935)
- February 12 – Lou Andreas-Salomé, Russian-born author (d. 1937)
- February 15 – Charles Édouard Guillaume, French physicist, Nobel Prize laureate (d. 1938)
- February 17 – Princess Helena, Duchess of Albany (d. 1922)
- February 19 – Henry Horne, 1st Baron Horne, British general (d. 1929)
- February 22 – Katō Tomosaburō, Imperial Japanese Navy officer, 12th Prime Minister of Japan (d. 1923)
- February 26 – King Ferdinand of Bulgaria (d. 1948)
- February 27 – Rudolf Steiner, Austrian philosopher, social reformer and author (d. 1925)
- March 2 – Nikola Ivanov, Bulgarian general (d. 1940)
- March 12 – József Konkolics, Hungarian Slovene writer (d. 1941)
- March 21 – Charles Swickard, German-American film director (d. 1929)
- April 2 – Iván Persa, Hungarian Slovene writer, Catholic priest (d. 1935)
- April 6 – Stanislas de Guaita, French poet (d. 1897)
- April 8 – Son Byong-hi, Korean independence activist (d. 1922)
- April 15 – Bliss Carman, Canadian poet (d. 1929)
- April 22 – István Tisza, 2-time Prime Minister of Hungary (d. 1918)
- April 23
- April 24 – Hedda Andersson, Swedish physician (d. 1950)
- May 5 – Peter Cooper Hewitt, American electrical engineer, inventor (d. 1921)
- May 7 – Rabindranath Tagore, Indian writer, Nobel Prize laureate (d. 1941)
- May 11 – Frederick Russell Burnham, American scouter(d. 1947)
- May 14 – Harro Magnussen, German sculptor (d. 1908)
- May 16 – Herman Webster Mudgett (alias H. H. Holmes), American serial killer (d. 1896)
- May 24 – Gerald Strickland, 4th Prime Minister of Malta, 23rd Governor of New South Wales, 15th Governor of Western Australia and 9th Governor of Tasmania (d. 1940)
- June 2 – Helen Herron Taft, First Lady of the United States (d. 1943)
- June 12 – William Attewell, English cricketer (d. 1927)
- June 19 – Doctor José Rizal, Philippine national hero (d. 1896)
- June 20 – Frederick Gowland Hopkins, English biochemist, recipient of the Nobel Prize in Physiology or Medicine (d. 1947)
- June 22 – Maximilian von Spee, German admiral (d. 1914)
- July 10 – Alice Stevenson, British supercentenarian (d. 1973)
- July 14 – Kate M. Gordon, American suffragette (d. 1932)
- August 4 – Henry Head, English neurologist (d. 1940)
- August 6 – Edith Roosevelt, First Lady of the United States (d. 1948)
- August 7 – Spencer S. Wood, United States Navy rear admiral (d. 1940)
- August 10 – Almroth Wright, British bacteriologist, immunologist (d. 1947)
- September 2 – Henrietta Crosman, American stage, film actress (d. 1944)
- September 10 – Niels Hansen Jacobsen, Danish sculptor, ceramist (d. 1941)
- September 11
- September 23
- September 28 – Wilhelm Diegelmann, German actor (d. 1934)
- September 30 – William Wrigley, Jr., American chewing gum industrialist (d. 1932)
- October 4 – Frederic S. Remington, American cowboy artist, sculptor (d. 1909)
- October 10 – Fridtjof Nansen, Norwegian explorer, scientist, and humanitarian, Nobel Prize laureate (d. 1930)
- October 16
- October 24 – Alexey Kaledin, Russian general (d. 1918)
- October 30 – Antoine Bourdelle, French sculptor (d. 1929)
- November 4 – Dimitrios Ioannou, Greek general (d. 1926)
- November 6 – James Naismith, Canadian inventor of basketball (d. 1939)
- November 11 – Elsa Eschelsson, Swedish scholar (d. 1911)
- December 4
- December 5 – Armando Diaz, Italian general, Marshal of Italy (d. 1928)
- December 8
- December 10 – Fridtjof Nansen, Norwegian explorer, recipient of the Nobel Peace Prize (d. 1930)
- December 15
- December 16 – Antonio de La Gándara, French painter (d. 1917)
- December 20 – Ivana Kobilca, Slovenian painter (d. 1926)
- December 29 – Kurt Hensel, German mathematician (d. 1941)
- January 2 – King Frederick William IV of Prussia (b. 1795)
- January 17 – Lola Montez, Irish-born dancer, mistress of King Ludwig I of Bavaria (b. 1821)
- January 19 – Albert Niemann, German chemist (b. 1834)
- March 10 – Taras Shevchenko, Ukrainian poet (b. 1814)
- March 16 – Princess Victoria, Duchess of Kent and Strathearn, mother of Queen Victoria (b. 1786)
- April 15 – Isaiah Stillman, U.S. Army Major in the Black Hawk War (b. 1793)
- May 29 – Joachim Lelewel, Polish nationalist historian (b. 1786)
- June 3 – Stephen A. Douglas, American Senator from Illinois, Democratic Presidential candidate (b. 1813)
- June 6 – Camillo Benso, Count of Cavour, 1st Prime Minister of Italy (b. 1810)
- June 13 – Richard Lawrence, failed assassin of Andrew Jackson (b. 1800)
- June 25 – Abdülmecid I, Ottoman Sultan (b. 1823)
- June 29 – Elizabeth Barrett Browning, English poet (b. 1806)
- July 22 – Barnard Elliott Bee, Jr., Confederate general (b. 1824)
- July 25 – Jonas Furrer, member of the Swiss Federal Council (b. 1805)
- August 10 – Nathaniel Lyon, first Union Army General to die in combat in the American Civil War (b. 1818)
- August 12 – Eliphalet Remington, American gunsmith, founder of Remington Arms (b. 1793)
- August 17 – Alcée Louis la Branche, American politician (b. 1806)
- August 22 – Xianfeng Emperor, 9th emperor of the Qing Dynasty (b. 1831)
- August 24 – Pierre Berthier, French geologist (b. 1782)
- August 28 – William Lyon Mackenzie, Scottish journalist, 1st Mayor of Toronto (b. 1795)
- September 7 – Willie Person Mangum, American politician (b. 1792)
- October 5 – Antoni Melchior Fijałkowski, Polish bishop (b. 1778)
- October 26 – Edward "Ned" Kendall, American bandleader, instrumentalist (keyed bugle) (b. 1808)
- October 31 – Guillermo (William) Miller, English-born military leader in Peru (b. 1795)
- November 7 – Isobel Gunn, Scottish business person (b. 1780)
- November 11 – King Pedro V of Portugal (b. 1837)
- December 14 – Prince Albert, husband of Queen Victoria (b. 1819)
- December 18 – Ernst Anschütz, German teacher, organist, poet, and composer (b. 1780)