1855 (MDCCCLV) 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 1855th year of the Common Era (CE) and Anno Domini (AD) designations, the 855th year of the 2nd millennium, the 55th year of the 19th century, and the sixth year of the 1850s decade. As of the start of 1855, the Gregorian calendar was 12 days ahead of the Julian calendar, which remained in localised use until 1918.
- January 1 – Ottawa, Ontario is incorporated as a city.
- January 5 – Ramón Castilla begins his third term as President of Peru.
- January 23
- 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 Ocean and Pacific Ocean.
- 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. Missourians cross the border in large numbers to elect a pro-slavery body.
- April 3 – The Nepalese invasion of Tibet starts the Nepalese–Tibetan War (1855-1856).
- May 1 – Van Diemen's Land separated administratively from New South Wales and granted self-government.
- May 15
- May 17 – The Mount Sinai Hospital, New York, is dedicated (as the Jews' Hospital) in New York City; it opens to patients on June 5.
- May 22 – Province of Victoria separated administratively from New South Wales.
- June 15 – Stamp duty is removed from newspapers in Britain, creating mass media in the United Kingdom.
- June 29 – The Daily Telegraph newspaper begins publication in London.
- July 1 – Quinault Treaty signed: Quinault and Quileute cede their land to the United States.
- 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 – Australian Colonies granted self-governing status by the United Kingdom.
- August 1 – First ascent of Monte Rosa, the second highest summit in the Alps.
- September 3 – The last Bartholomew Fair is held in London, England.
- September 9 (August 28 O.S.) – Siege of Sevastopol (1854–55) (Crimean War): Sevastopol falls to French and British troops.
- September 27 – Alfred Tennyson reads from his new book Maud and additional 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 – Iloilo was opened to world trade by Queen Isabella II of Spain .
- October 17 – Henry Bessemer files his patent in the United Kingdom for the Bessemer process of steelmaking.
- October 24 – Van Diemen's Land 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.
- December 22 – The Metropolitan Board of Works is established in London.
- The cocaine alkaloid is first isolated by the German chemist Friedrich Gaedcke.
- Palm oil sales from West Africa to the United Kingdom reach 40,000 tons.
- Sual (present-day Pangasinan) and Zamboanga opened to world trade.
- January 5 – King Camp Gillette, American razor inventor (d. 1932)
- January 20 – Ernest Chausson, French composer (d. 1899)
- January 21
- February 4 – George Cope, American painter (d. 1929)
- February 12 – Marie-Anne de Bovet, French writer
- February 17 – Otto Liman von Sanders, German general (d. 1929)
- February 20 – John R. Lindgren, founder of the banking firm Haugan & Lindgren (d. 1915)
- March 4 – Luther Emmett Holt, American paediatrician (d. 1924)
- March 13 – Percival Lowell, American astronomer (d. 1916)
- March 24 – Andrew W. Mellon, American banker and philanthropist (d. 1937)
- April 9
- April 21 – Hardy Richardson, nineteenth century baseball player (d. 1931)
- April 27 – Caroline Rémy de Guebhard, French feminist (d. 1929)
- 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 – Sri Yukteswar Giri, Author of The Holy Science
- May 21 – Emile Verhaeren, Belgian poet (d. 1916)
- May 23 – Isabella Ford, English socialist, feminist, trade unionist and writer (d. 1924)
- June 2 – Archibald Berkeley Milne, British admiral (d. 1938)
- June 28 – Theodor Reuss, German occultist (d. 1923)
- 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 and women's right activist (d. 1907)
- 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)
- October 21 – Howard Hyde Russell, American activist (d. 1946)
- October 24 – James S. Sherman, 27th Vice President of the United States (d. 1912)
- November 1 – Templin Potts, American naval officer; eleventh Naval Governor of Guam (d. 1927)
- November 5 – Léon Teisserenc de Bort, French meteorologist (d. 1913)
- November 6 – Ezra Seymour Gosney, American philanthropist and eugenicist (d. 1942)
- December 17 – Frank Hedges Butler, British wine merchant and founding member of the Aero Club of Great Britain (d. 1928)
- December 28 – John William Wood, Sr., North Carolinan politician, founder of Benson, North Carolina (d. 1928)
- December 29 – William Thompson Sedgwick, American teacher, epidemiologist and bacteriologist (d. 1921)
- January 6 – Giacomo Beltrami, Italian explorer (b. 1779)
- January 10 – Mary Russell Mitford, English novelist and dramatist (b. 1787)
- January 15 – Henri Braconnot, French chemist and pharmacist (b. 1780)
- 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 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 – 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, first Baron Raglan, commander of British forces in the Crimean War (b. 1788)
- July 12 (June 30 O.S.) – Pavel Nakhimov, Russian admiral (b. 1802)
- August 7 – Mariano Arista, President of Mexico (b. 1802)
- August 30 – Feargus O'Connor, British political radical and 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 and writer (b. 1798)
- December 6 – William John Swainson, English naturalist and artist (b. 1789)