January – John Tyndall first explains the workings of the greenhouse effect.
Abraham Lincoln signs the Emancipation Proclamation during the third year of the American Civil War, making the abolition of slavery in the Confederate states an official war goal. It proclaims the freedom of 3.1 million of the nation's four million slaves, and immediately frees 50,000 of them, with the rest freed as Union armies advance.
The first claim under the Homestead Act is made, for a farm in Nebraska.
January 3 – Harper's Weekly publishes Thomas Nast's first drawing of the modern Santa Claus (although Santa existed previously).
January 4 – The New Apostolic Church, a Christian and chiliastic church, established in Hamburg, Germany.
January 7 – In the Swiss canton Ticino, the village of Bedretto is partly destroyed, and 29 killed, by an avalanche.
Ground is broken in Sacramento, California, on the construction of the First Transcontinental Railroad in the United States.
The Yorkshire County Cricket Club is founded at the Adelphi Hotel in Sheffield, England.
January 10 – The first section of the London Underground Railway (Paddington to Farringdon Street) opens officially.
American Civil War – Battle of Arkansas Post: General John McClernand and Admiral David Dixon Porter capture the Arkansas River for the Union.
In the Swiss Canton Ticino, the roof of the church of Sant'Antonio in Locarno collapses under the weight of snow, killing 47.
January 15 – French intervention in Mexico – French forces bombard Veracruz.
January 21 – Adam Opel founds Opel AG.
January 22 – The January Uprising breaks out in Poland, Lithuania and Belarus. The aim of the national movement is to liberate the Polish–Lithuanian–Ruthenian Commonwealth from Russian occupation.
January 29 – American Indian Wars: The United States Army led by General Patrick Edward Connor massacres Chief Bear Hunter and forces of the Shoshone in the Bear River Massacre in Idaho Territory.
January 31 – Jules Verne's first adventure novel, Five Weeks in a Balloon (Cinq semaines en ballon), is published in Paris.
February 1 – Radicals in Lithuania, Belarus, Latvia, northern Ukraine and western Russia join the January Uprising.
February 2 – January Uprising: Polish peasants are massacred by Russian hussars at Čysta Būda, near Marijampolė.
February 7 – HMS Orpheus sinks attempting to enter Manukau Harbour in New Zealand, with the loss of 189 lives.
The world-famous midgets General Tom Thumb and Lavinia Warren get married in New York City; P. T. Barnum takes an entrance fee.
Alanson Crane patents a fire extinguisher.
February 16 – Kansas State Agricultural College is established as the first land grant college created under the 1862 Morrill Act.
February 17 – First meeting of the "Committee of the Five" in Geneva, Switzerland, regarded as the foundation of the International Committee of the Red Cross, following the lead of humanitarian businessman Henry Dunant.
February 24 – Arizona is organized as a United States territory.
February 26 – Abraham Lincoln signs the National Banking Act into law.
March 2 – Clapham Junction railway station opens in London.
Idaho Territory is organized by the U.S. Congress.
The U.S. National Conscription Act is signed, leading to the New York City draft riots in July.
March 10 – Albert Edward, Prince of Wales (later Edward VII) marries Princess Alexandra of Denmark (later Queen Alexandra).
March 14 – Queen Victoria issues Letters Patent granting Goulburn, New South Wales, city status, making it Australia's first inland city.
March 19 – The SS Georgiana is destroyed on her maiden voyage while attempting to run the blockade into Charleston, South Carolina. The wreck is discovered exactly 102 years later by E. Lee Spence.
March 30 – 17-year-old Prince Wilhelm George of Denmark is elected by the Hellenic Parliament as George, King of the Hellenes; he will reign in Greece for fifty years. He arrives in Athens on October 30 to take the throne.
April 14 – Treaty of Huế is signed between Vietnam and the French Empire.
April 17–May 2 – The Grierson's Raid: Union cavalrymen are ambushed while crossing the Tickfaw river in Mississippi.
April 20 – American Civil War – The Battle of Washington ends inconclusively in Beaufort County, North Carolina.
Ayyam-i butun: Bahá'u'lláh begins a 12-day stay in the Najibiyyih gardens, Baghdad (now known as the Garden of Ridván) during which he declares his station as "He whom God shall make manifest". This date is celebrated in the Bahá'í Faith as the festival of Ridván.
January Uprising: The Polish peasant army, now led by Zygmunt Sierakowski, achieves its first victory over the Russian army, near Raguva.
April 30 – Battle of Camarón in Mexico: 65 soldiers of the French Foreign Legion fight 2,000 Mexicans.
May 1–4 – American Civil War – The Battle of Chancellorsville: General Robert E. Lee defeats Union forces with 13,000 Confederate casualties, among them Stonewall Jackson (lost to friendly fire), and 17,500 Union casualties.
Granadine Confederation becomes the United States of Colombia under President Tomás Cipriano de Mosquera.
January Uprising: The Polish insurgent army is defeated by the Russians near Gudiškis.
May 14 – American Civil War – Battle of Jackson, Mississippi: Union General Ulysses S. Grant defeats Confederate General Joseph E. Johnston, opening the way for the Siege of Vicksburg.
After a 2-month siege, the French army of Bazaine takes Puebla, Mexico.
Opening of Salon des Refusés in Paris draws attention to paintings by avant-garde artists, notably Manet's Le Déjeuner sur l’herbe.
May 18 – American Civil War: The Siege of Vicksburg begins (ends July 4, when 30,189 Confederate men surrender).
American Civil War: The Siege of Port Hudson, Louisiana by Union forces begins.
The General Conference of Seventh-day Adventists is formed in Battle Creek, Michigan.
May 23 – Ferdinand Lassalle founds the Allgemeiner Deutscher Arbeiterverein (General German Workers' Association, ADAV), the first socialist workers party in Germany.
May 28 – American Civil War – The 54th Massachusetts, the first African-American regiment, leaves Boston to fight for the Union.
May 31 – The first Prix de l'Arc de Triomphe horse race is held.
June 7 – French intervention in Mexico – French forces enter Mexico City.
June 9 – American Civil War – The Battle of Brandy Station, Virginia ends inconclusively.
June 12 – The Arts Club is founded by Charles Dickens, Frederic Leighton and others in Hanover Square, London.
June 13 – Samuel Butler's dystopian article "Darwin among the Machines" is published (as by "Cellarius") in The Press newspaper in Christchurch, New Zealand; it will be incorporated into his novel Erewhon (1872).
June 14 – American Civil War – Second Battle of Winchester: A Union garrison is defeated by the Army of Northern Virginia in the Shenandoah Valley town of Winchester, Virginia.
June 17 – American Civil War – The Battle of Aldie in the Gettysburg Campaign ends inconclusively.
June 20 – West Virginia is admitted as the 35th U.S. state.
July – The tiny Confederate States of America hand-propelled submarine H. L. Hunley is first tested successfully.
July 1 – Slavery is abolished in the Dutch colonies of Suriname (independent from 1975) and Curaçao and Dependencies.
July 1–3 – American Civil War: Battle of Gettysburg – Union forces under George G. Meade turn back a Confederate invasion by Robert E. Lee in the largest battle of the war (28,000 Confederate casualties, 23,000 Union).
July 4 – American Civil War: Siege of Vicksburg – Ulysses S. Grant and the Union army capture the Confederate city Vicksburg, Mississippi, after the town surrenders. The siege lasts 47 days.
July 6 – Queen Victoria issues Letters Patent annexing to South Australia the part of the colony of New South Wales that will later become the Northern Territory.
July 9 – The Siege of Port Hudson ends and the Union controls the entire Mississippi River for the first time.
July 13 – American Civil War – New York City draft riots: In New York City, opponents of conscription begin 3 days of violent rioting, which will be regarded as the worst in the history of the United States with around 120 killed.
July 16 – Battle of Shimonoseki Straits: The screw sloop USS Wyoming engages with the Chōshū Domain fleet before withdrawing, Japan's first naval engagement between elements of modern navies.
July 17 – New Zealand Wars against the Māori people resume as British forces in New Zealand led by Duncan Cameron begin their Invasion of the Waikato.
July 17 – Battle of Honey Springs was a Union strategic victory over the Confederates for control of Indian Territory north of the Arkansas River.
July 18 – American Civil War: The first formal African American military unit, the 54th Massachusetts Volunteer Infantry, unsuccessfully assaults Confederate-held Fort Wagner but their valiant fighting still proves the worth of African American soldiers during the war. Their commander, Colonel Robert Shaw is shot leading the attack and was buried with his men (450 Union, 175 Confederate).
July 26 – American Civil War – Morgan's Raid: At Salineville, Ohio, Confederate cavalry leader John Hunt Morgan and 375 of his volunteers are captured by Union forces.
July 30 – American Indian Wars: Representatives of the United States and tribal leaders including Chief Pocatello (of the Shoshone) sign the Treaty of Box Elder.
August 3 – Otago Boys' High School founded in New Zealand.
August 8 – American Civil War: Following his defeat in the Battle of Gettysburg, General Robert E. Lee sends a letter of resignation to Confederate President Jefferson Davis (Davis refuses the request upon receipt).
August 15–17 – Bombardment of Kagoshima: The British Royal Navy bombards the town of Kagoshima in Japan in retribution after the Namamugi Incident of 1862.
August 16 – After Spain's annexation of the Dominican Republic, rebels raise the Dominican flag in Santiago to begin the Dominican Restoration War.
August 17 – American Civil War: In Charleston, South Carolina, Union batteries and ships bombard Confederate-held Fort Sumter (the bombardment does not end until December 31).
August 21 – American Civil War
Battle of Lawrence: Lawrence, Kansas is attacked by William Quantrill's raiders, who kill an estimated 200 men and boys. The raid becomes notorious in the North as one of the most vicious atrocities of the Civil War.
American clipper Anglo Saxon westbound is captured and burned by Confederate privateer Florida off the south coast of Ireland.
September – The Western Railroad from Fayetteville, North Carolina to the coal fields of Egypt, North Carolina is completed.
September 6 – American Civil War: Confederates evacuate Battery Wagner and Morris Island in South Carolina.
September 16 – Robert College of Istanbul–Turkey, the first American educational institution outside the United States, is founded by Christopher Robert, an American philanthropist.
September 19–20 – American Civil War: Confederate forces turn back a Union invasion of Georgia in the Battle of Chickamauga.
September 30 – Georges Bizet's opera Les pêcheurs de perles debuts at the Théâtre Lyrique in Paris.
October 3 – US President Lincoln proclaims a national Thanksgiving day to be celebrated the final Thursday in November in the United States.
October 5 – The Brooklyn, Bath and Coney Island Rail Road starts operations in Brooklyn, New York; this is now the oldest right-of-way on the New York City Subway, the largest rapid transit system in the United States and one of the largest in the world.
October 14 – American Civil War: Battle of Bristoe Station – Confederate General Robert E. Lee forces fail to drive the Union army out of Virginia.
October 15 – American Civil War: The Confederate submarine H. L. Hunley sinks during a test, killing Horace Lawson Hunley (its inventor) and a crew of seven.
October 23 – Ffestiniog Railway in North Wales introduces steam locomotives into general service, the first time this has been done anywhere in the world on a public railway of such a narrow gauge (2 ft (60 cm)).
October 26 – The Football Association is formed in London.
October 26–29 – The Resolutions of the Geneva International Conference are signed by sixteen countries meeting in Geneva agreeing to form the International Red Cross.
October 29 – American Civil War – Battle of Wauhatchie: Forces under Union General Ulysses S. Grant, having fought through the night, ward off a Confederate attack led by General James Longstreet. Union forces thus open a supply line into Chattanooga, Tennessee.
November 4 – Hector Berlioz's opera Les Troyens debuts at the Théâtre Lyrique in Paris.
November 15 – The death of King Frederick VII of Denmark and his succession by his distant cousin Christian IX marks the beginning of the Second Schleswig-Holstein crisis.
November 16 – American Civil War – Battle of Campbell's Station: Near Knoxville, Tennessee, Confederate troops led by General James Longstreet unsuccessfully attack Union forces under General Ambrose Burnside.
November 17 – American Civil War – The Siege of Knoxville begins: Confederate forces led by General James Longstreet place Knoxville, Tennessee under siege (the 2-week-long siege and an attack are unsuccessful).
November 18 – King Christian IX of Denmark signs the November constitution, which declares Schleswig to be part of Denmark, regarded by the German Confederation as a violation of the London Protocol of 1852, leading to the German–Danish war of 1864.
November 19 – American Civil War: U. S. President Abraham Lincoln delivers the Gettysburg Address at the military cemetery dedication ceremony in Gettysburg, Pennsylvania.
November 23 – American Civil War – The Battle of Chattanooga III begins: Union forces led by General Ulysses S. Grant reinforce troops at Chattanooga, Tennessee and counter-attack Confederate troops.
November 24 – American Civil War – Battle of Lookout Mountain: Near Chattanooga, Tennessee, Union forces under General Ulysses S. Grant capture Lookout Mountain and begin to break the Confederate siege of the city led by General Braxton Bragg.
November 25 – American Civil War – Battle of Missionary Ridge: At Missionary Ridge in Tennessee, Union forces led by General Ulysses S. Grant break the Siege of Chattanooga by routing Confederate troops under General Braxton Bragg.
November 26 – American Civil War – Mine Run: Union forces under General George Meade position against troops led by Confederate General Robert E. Lee (Meade's forces can not find any weaknesses in the Confederate lines and give up trying after 5 days).
November 27 – American Civil War: Confederate cavalry leader John Hunt Morgan and several of his men escape the Ohio state prison, and return safely to the South.
December 1 – Opening of first steam-operated passenger railway in New Zealand, at Christchurch in South Island.
Church of the Company Fire in Santiago, Chile, kills between 2,000 and 3,000.
The Football Association laws are agreed.
Romania opens its first mountain railway (from Anina to Oravița).
22-year-old Gerard Adriaan Heineken buys the brewery 'De Hooiberg' ("The Haystack") in Amsterdam.
Linoleum patented in the United Kingdom.
Association football is played for the first time in the modern era.
December 25 – Henry Wadsworth Longfellow writes the poem "Christmas Bells", or, as it is better known "I Heard the Bells on Christmas Day".
Beginning of Second Anglo-Ashanti war.
Chōshū Five leave Japan secretly to study at University College London, part of the ending of sakoku.
Bartolomé Mitre secretly backs the revolt of Venancio Flores against the Uruguayan Blanco government
Douglas becomes the capital of the Isle of Man, after its parliament (Tynwald) moves its chambers from Castletown.
First outbreak of phylloxera on the European mainland observed in the vineyards of the southern Rhône region of France.
Richard Owen publishes the first description of a fossilised bird, Archaeopteryx.
Winged Victory of Samothrace found at Samothrace by Charles Champoiseau. Made c.190 BC(?), it is now in the Musée du Louvre, Paris.
January 1 – Pierre de Coubertin, French founder of the modern Olympic Games (d. 1937)
January 12 – Swami Vivekananda, Indian religious leader (d. 1902)
January 14 – Gomes da Costa, Portuguese general, who served as the 10th President of Portugal (d. 1929)
January 15 – Wilhelm Marx, Chancellor of Germany (d. 1946)
David Lloyd George, Prime Minister of the United Kingdom (d. 1945)
Constantin Stanislavski, Russian theatre practitioner and founder of modern realistic acting (d. 1938)
January 28 – Ernest William Christmas, Australian painter (d. 1918)
February 11 – John F. Fitzgerald, Mayor of Boston (d. 1950)
April 29 – Signe Häggman, Finnish physical education pioneer (d. 1911)
March 1 – Sydney Deane, Australian cricketer and actor (d. 1934)
March 11 – Andrew Stoddart, English cricketer (d. 1915)
March 12 – Gabriele D'Annunzio, Italian writer, war hero, and politician (d. 1938)
March 25 – Simon Flexner, American pathologist (d. 1946)
May 21 – Archduke Eugen of Austria, Austrian field marshal (d. 1954)
March 27 – Henry Royce, English automobile pioneer (d. 1933)
April 15 – Ioannis Svoronos, Greek numismatist (d. 1922)
April 18 – Count Leopold Berchtold, Austro-Hungarian foreign minister (d. 1942)
April 28 – Josiah Thomas, Australian politician (d. 1933)
April 29 – William Randolph Hearst, American newspaper publisher (d. 1951)
May 24 – George Grey Barnard, American sculptor (d. 1938)
May 29 – Arthur Mold, English cricketer (d. 1921)
June 2 – Felix Weingartner, Yugoslavian conductor (d. 1942)
June 13 – Lucy, Lady Duff-Gordon, English fashion designer (d. 1942)
June 17 – Charles Michael, Duke of Mecklenburg, head of the House of Mecklenburg-Strelitz (d. 1934)
July 1 – William Grant Stairs, Canadian explorer (d. 1892)
July 4 – Hugo Winckler, German archaeologist and historian who uncovered the capital of the Hittite Empire (Hattusa) (d. 1913)
July 6 – Reginald McKenna, British Chancellor of the Exchequer 1915–1916 (d. 1943)
July 21 – C. Aubrey Smith, English actor (d. 1948)
July 25 – Alison Skipworth, English actress (d. 1952)
July 30 – Henry Ford, American automobile manufacturer and industrialist (d. 1947)
August 3 – Géza Gárdonyi, Hungarian author (d. 1922)
August 11 – Gaston Doumergue, President of France during the Third Republic (d. 1937)
August 17 – Gene Stratton-Porter, American author, screenwriter and naturalist (d. 1924)
September 1 – João Pinheiro Chagas, Prime Minister of Portugal (d. 1925)
September 9 – Herbert Henry Ball, Ontario Politician and King's Printer (d. 1943)
Arthur Henderson, Scottish politician, recipient of the Nobel Peace Prize (d. 1935)
Franz von Hipper, German admiral (d. 1932)
September 21 – John Bunny, American film comedian (d. 1915)
Alexandre Yersin, Swiss-French physician and bacteriologist (d. 1943)
G. R. S. Mead, British writer (d. 1933)
September 30 – Reinhard Scheer, German admiral (d. 1928)
October 1 – Adolph Otto Niedner, American cartridge designer (d. 1954)
October 7 – Clarence Stewart Williams, American admiral (d. 1951)
Lionel Cripps, Rhodesian politician (d. 1950)
Louis Cyr, Canadian strongman (d. 1912)
October 16 – Austen Chamberlain, English statesman, recipient of the Nobel Peace Prize (d. 1937)
October 18 – Tommy Tucker, 19th century baseball pioneer (d. 1935)
November 8 – Eero Järnefelt, Finnish realist painter (d. 1937)
November 11 – Paul Signac, French Neo-Impressionist painter (d. 1935)
November 14 – Leo Baekeland Belgian-born American chemist (d. 1944)
November 20 – Zeffie Tilbury, stage & film actress(daughter of Lydia Thompson) (d. 1950)
November 30 – Andrés Bonifacio, Filipino revolutionary (d. 1897)
December 5 – Pattillo Higgins, oil pioneer and businessman, known as the "Prophet of Spindletop" (d. 1955)
Felix Calonder, Swiss politician (d. 1952)
Richard Warren Sears, American businessman (d. 1914)
Pietro Mascagni, Italian composer (d. 1945)
December 8 – Albert Abrams, American doctor (d. 1924)
Georg Bruchmüller, influential German artillery officer (d. 1948)
Annie Jump Cannon, American astronomer (d. 1941)
Edvard Munch, Norwegian painter (d. 1944)
Sahibzada Abdul Qayyum Khan, educationist, politician British India
December 16 – George Santayana, philosopher, poet, essayist and novelist (d. 1952)
December 18 – Archduke Franz Ferdinand of Austria (d. 1914)
Qasim Amin, writer (d. 1908)
Black Elk/ Heȟáka Sápa', Oglala Teton Lakota (Western Sioux) medicine/holy man (d. 1950)
María Abella de Ramírez, Uruguayan women's rights activist (d. 1926)
Aaron S. Watkins, American Presidential candidate (d. 1941)
Kenneth Robert Balfour, British Conservative Party politician (d. 1936)
April 1 – Jakob Steiner, Swiss mathematician (b. 1796)
April 10 – Giovanni Battista Amici, Italian astronomer, microscopist, and botanist (b. 1786)
April 21 – Sir Robert Bateson, 1st Baronet, Irish nobility (b. 1782)
May 7 – Earl Van Dorn, American Confederate general (b. 1820)
May 10 – Thomas J. "Stonewall" Jackson, American Confederate general (b. 1824)
June 7 – Antonio Valero de Bernabé, Latin American liberator (b. 1790)
June 9 – Dost Mohammad Khan, Emir of Kabul, King of Kandahar (b. 1793)
June 26 – Andrew Hull Foote, American admiral (b. 1806)
July 1 – John F. Reynolds, American general (b. 1820)
July 5 – Lewis A. Armistead, Confederate general (b. 1817)
July 21 - Josephine Kablick, Czech botanist and paleontologist (b. 1787)
July 26 – Sam Houston, first President of the Republic of Texas (b. 1793)
August 13 – Eugène Delacroix, French painter (b. 1798)
September 17 – Alfred de Vigny, French author (b. 1797)
September 20 – Jacob Grimm, German folklorist (b. 1785)
October 13 – Philippe Antoine d'Ornano, Marshal of France (b. 1784)
November 2 – Theodore Judah, American railroad engineer (b. 1826)
November 15 – King Frederick VII of Denmark (b. 1808)
December 2 – Jane Pierce, 15th First Lady of the United States (b. 1806)
December 13 – Christian Friedrich Hebbel, German writer (b. 1813)
December 16 – John Buford, American general (b. 1826)
December 24 – William Makepeace Thackeray, British novelist (b. 1811)
The film Glory (starring Morgan Freeman, Denzel Washington, and Matthew Broderick), shows events of 1863, notably the assault on Fort Wagner.
1863 (MDCCCLXIII) was a common year starting on Thursday (dominical letter D) of the Gregorian calendar and a common year starting on Tuesday (dominical letter F) of the Julian calendar, the 1863rd year of the Common Era (CE) and Anno Domini (AD) designations, the 863rd year of the 2nd millennium, the 63rd year of the 19th century, and the 4th year of the 1860s decade. As of the start of 1863, the Gregorian calendar was 12 days ahead of the Julian calendar, which remained in localized use until 1923.