1855

January–March

 * January 1 – Ottawa, Ontario, is incorporated as a city.
 * January 5 – Ramón Castilla begins his third term as President of Peru.
 * January 23
 * The first bridge over the Mississippi River opens in modern-day Minneapolis, a predecessor of the Father Louis Hennepin Bridge.
 * The 8.2–8.3 Wairarapa earthquake claims between five and nine lives near the Cook Strait area of New Zealand.
 * January 26 – The Point No Point Treaty is signed in the Washington Territory.
 * January 27 – The Panama Railway becomes the first railroad to connect the Atlantic and Pacific Oceans.
 * January 29 – Lord Aberdeen resigns as Prime Minister of the United Kingdom, over the management of the Crimean War.
 * February 5 – Lord Palmerston becomes Prime Minister of the United Kingdom.
 * February 11 – Kassa Hailu is crowned Tewodros II, Emperor of Ethiopia.
 * February 12 – Michigan State University (the "pioneer" land-grant college) is established.
 * February 15 – The North Carolina General Assembly incorporates the Western North Carolina Railroad, to build a rail line from Salisbury to the western part of the state.
 * February 22 – Pennsylvania State University is founded, as the Farmers' High School of Pennsylvania.
 * March 2 – Alexander II ascends the Russian throne, upon the death of his father Nicholas I.
 * March 3 – The United States Congress appropriates $30,000 to create the U.S. Camel Corps.
 * March 16 – Bates College is founded by abolitionists in Lewiston, Maine.
 * March 17 – Taiping Rebellion: A Taiping army of 350,000 invades Anhui.
 * March 30 – Elections are held for the first Kansas Territory legislature. Missourian 'Border Ruffians' cross the border in large numbers to elect a pro-slavery body.

April–June

 * April 3 – The Nepalese invasion of Tibet starts the Nepalese–Tibetan War (1855–1856).
 * April 7 – Battle of Kaba: The Kingdom of Tonga intervenes in the war between the self-proclaimed Tui Viti (King) of Fiji, Cakobau, and his rivals the Confederation of Rewa resulting in Rewa's defeat and the tenuous unification of Fiji under Cakobau.
 * April 18 – The Bordeaux Wine Official Classification of 1855 is first listed.
 * May 1 – Van Diemen's Land is separated administratively from New South Wales and granted self-government.
 * May 3 – American adventurer William Walker and a group of mercenaries sail from San Francisco to conquer Nicaragua.
 * May 15
 * The Exposition Universelle officially opens in Paris (a direct result of the exhibition is the introduction of the Bordeaux Wine Official Classification of 1855).
 * The Great Gold Robbery is made from a train between London Bridge and Folkestone in England.
 * May 17 – Mount Sinai Hospital, New York, is dedicated (as the Jews' Hospital) in New York City; it opens to patients on June 5.
 * May 22 – The province of Victoria is separated administratively from New South Wales.
 * June 15 – Stamp duty is removed from British newspapers, creating mass media in the United Kingdom.
 * June 29 – The Daily Telegraph newspaper begins publication in London.

July–September

 * July – Bank of Toronto incorporated in Canada (in 1955 it will merge with The Dominion Bank to become Toronto-Dominion Bank).
 * July 1 – The Quinault Treaty, in which the Quinault and Quileute tribes cede their land to the United States, is signed.
 * July 2 – The Kansas territorial legislature convenes in Pawnee, and begins passing proslavery laws.
 * July 4 – Walt Whitman's poetry collection Leaves of Grass is published in Brooklyn.
 * July 16 – The Australian Colonies are granted self-governing status by the United Kingdom.
 * August 1 – Monte Rosa, the second-highest summit in the Alps, is first ascended.
 * September 3 – The last Bartholomew Fair is held in London, England.
 * September 9 (August 28 O.S.) – Crimean War: Siege of Sevastopol (1854–1855) – Sevastopol falls to French and British troops.
 * September 27 – Alfred Tennyson reads from his new book Maud and other poems, at a social gathering in the home of Robert and Elizabeth Browning in London; Dante Gabriel Rossetti makes a sketch of him doing so.
 * September 29 – The Port of Iloilo in the Philippines is opened to international trade, by Queen Isabel II of Spain. This year also the ports of Sual (modern-day Pangasinan) and Zamboanga City are opened to international trade.

October–December

 * October 17 – Henry Bessemer files his patent in the United Kingdom for the Bessemer process of steelmaking.
 * October 24 – Van Diemen's Land is officially renamed Tasmania.
 * November 17 – Scottish missionary explorer David Livingstone becomes the first European to see Victoria Falls, in modern-day Zambia–Zimbabwe.
 * November 21 – Large-scale Bleeding Kansas violence begins, with events leading to the 'Wakarusa War' between antislavery and proslavery forces.
 * November 10 – Henry Wadsworth Longfellow's fictional poem The Song of Hiawatha is published in Boston.
 * December 11 – Ignacio Comonfort (1812–1863) becomes President of Mexico.
 * December 22 – The Metropolitan Board of Works is established in London.

Date unknown

 * Samuel Colt incorporates his business as the Colt's Patent Firearms Manufacturing Company and opens a new factory, the Colt Armory, in Hartford, Connecticut.
 * The cocaine alkaloid is first isolated by German chemist Friedrich Gaedcke.
 * Palm oil sales from West Africa to the United Kingdom reach 40,000 tons.

January–June

 * January 5 – King C. Gillette, American razor inventor (d. 1932)
 * January 20 – Ernest Chausson, French composer (d. 1899)
 * January 21
 * John Browning, American firearms inventor (d. 1926)
 * Henry Jackson, British admiral (d. 1929)
 * February 6 – Barbara Galpin, American journalist (d. 1922)
 * February 12 – Marie-Anne de Bovet, French writer
 * February 13 – Paul Deschanel, President of France (d. 1922)
 * February 17 – Otto Liman von Sanders, German general (d. 1929)
 * February 24 – Johannes von Eben, German general (d. 1924)
 * March 4 – Luther Emmett Holt, American pediatrician (d. 1924)
 * March 12 – Eduard Birnbaum, Polish-born German cantor (d. 1920)
 * March 13 – Percival Lowell, American astronomer (d. 1916)
 * March 24 – Andrew Mellon, American banker, philanthropist (d. 1937)
 * March 25 – Grace Carew Sheldon, American journalist and businesswoman (d. 1921)
 * March 29 – James O. Barrows, American stage and screen actor (d.1925)
 * April 9
 * Pavlos Kountouriotis, Greek admiral, 2-time president (d. 1935)
 * John Marden, Australian headmaster, pioneer of women's education (d. 1924)
 * April 21 – Hardy Richardson, American baseball player (d. 1931)
 * April 23 – Marco Fidel Suárez, 9th President of Colombia (d. 1927)
 * April 27 – Caroline Rémy de Guebhard, French feminist (d. 1929)
 * April 28 – Mario Nicolis di Robilant, Italian general (d. 1943)
 * May 1 – Marie Corelli, English novelist (d. 1924)
 * May 8 – Bohuslav Brauner, Czech chemist (d. 1935)
 * May 9 – Julius Röntgen, German-Dutch classical composer (d. 1932)
 * May 10 – Swami Sri Yukteswar Giri, Bengali yogi, author of The Holy Science (d. 1936)
 * May 21
 * Émile Verhaeren, Belgian poet (d. 1916)
 * Ella Stewart Udall, American telegraphist (d. 1937)
 * May 23 – Isabella Ford, English socialist, feminist, trade unionist and writer (d. 1924)
 * May 28 – Emilio Estrada Carmona, 18th President of Ecuador (d. 1911)
 * June 1 – Edward Angle, American dentist (d. 1930)
 * June 2 – Archibald Berkeley Milne, British admiral (d. 1938)
 * June 14 – Robert M. La Follette, American politician (d. 1925)
 * June 18 – Alice Sudduth Byerly, American temperance activist (d. 1904)
 * June 28 – Theodor Reuss, German occultist (d. 1923)

July–December

 * July 26 – Ferdinand Tönnies, German sociologist (d. 1936)
 * August 25 – Hugo von Pohl, German admiral (d. 1916)
 * August 28 – Alexander Bethell, British admiral (d. 1932)
 * August 31 – Vsevolod Rudnev, Russian admiral (d. 1913)
 * September 5 – Henry Victor Deligny, French general (d. 1938)
 * September 8 – Marieta de Veintemilla, Ecuadorian first lady, women's rights activist (d. 1907)
 * September 9 – Houston Stewart Chamberlain, British-born German writer (d. 1927)
 * September 15 – Orishatukeh Faduma, Guyana-born African-American Christian missionary, educator and advocate for African culture (d. 1946)
 * September 17 – Effie Ellsler, American actress (d. 1942)
 * September 25 – James P. Parker, United States Navy commodore (d. 1942)
 * October 10 – Eduard von Capelle, German admiral (d. 1931)
 * October 12 – Arthur Nikisch, Hungarian conductor (d. 1922)
 * November 1 – Templin Potts, American naval officer; 11th Naval Governor of Guam (d. 1927)
 * November 5
 * Léon Teisserenc de Bort, French meteorologist (d. 1913)
 * Eugene V. Debs, American union leader (d. 1926)
 * November 6 – E. S. Gosney, American philanthropist, eugenicist (d. 1942)
 * November 8 – Nikolaos Triantafyllakos, Prime Minister of Greece (d. 1939)
 * December 16 – Alice Mary Dowd, American educator, poet (d. 1943)
 * December 29 – William Thompson Sedgwick, American teacher, epidemiologist and bacteriologist (d. 1921)

Date unknown

 * Florence Huntley, American humorist and occult author (d. 1912)
 * Flora Haines Loughead, American miner; mother of Allan Lockheed, founder of Lockheed aerospace company (d. 1943)
 * Katharine A. O'Keeffe O'Mahoney, Irish-born American teacher of poetry to Robert Frost (d. 1918)

January–June

 * January 6 – Giacomo Beltrami, Italian explorer (b. 1779)
 * January 8 – Diponegoro, Leader of Javanese Rebellion (b. 1785)
 * January 10 – Mary Russell Mitford, English novelist, dramatist (b. 1787)
 * January 15 – Henri Braconnot, French chemist, pharmacist (b. 1780)
 * January 17 – Shūsaku Narimasa Chiba, Japanese swordsman (b. 1792)
 * January 26 – Gérard de Nerval, French writer (b. 1808)
 * February 6 – Josef Munzinger, Member of the Swiss Federal Council (b. 1791)
 * February 23 – Carl Friedrich Gauss, German mathematician, astronomer, and physicist (b. 1777)
 * March 2 – Emperor Nicholas I of Russia (b. 1796)
 * March 6– Bandō Shūka I, Japanese Kabuki actor (b. 1813)
 * March 8 – William Poole, infamous member of New York City's Bowery Boys Gang (b. 1821)
 * March 29 – Henri Druey, member of the Swiss Federal Council (b. 1799)
 * March 31 – Charlotte Brontë, English author (b. 1816)
 * May 5 – Sir Robert Inglis, English politician (b. 1786)
 * May 23 – Charles Robert Malden, English explorer (b. 1797)
 * May 30 – Mary Reibey, Australian businesswoman (b. 1777)
 * June 7 – Friederike Lienig, Latvian entomologist (b. 1790)
 * June 28 – FitzRoy Somerset, 1st Baron Raglan, commander of British forces in the Crimean War (b. 1788)

July–December

 * July 12 (June 30 O.S.) – Pavel Nakhimov, Russian admiral (b. 1802)
 * August 7 – Mariano Arista, President of Mexico (b. 1802)
 * August 12 – Helen Hunt Jackson, American activist (b. 1830)
 * August 30 – Feargus O'Connor, British political radical, Chartist leader (b. 1794)
 * September 7 – William Barton Wade Dent, U.S. Congressman (b. 1806)
 * November 11 – Søren Kierkegaard, Danish philosopher (b. 1813)
 * September 20 – José Trinidad Reyes, Honduran Father, national hero, and founder of Autonomous National University of Honduras (b. 1797)
 * November 26 – Adam Mickiewicz, Lithuanian-Polish poet, writer (b. 1798)
 * December 6 – William John Swainson, English naturalist, artist (b. 1789)