|Covid-19|January 1 – Haiti gains independence from France and becomes the first black republic, having the only successful slave revolt ever.
February 14 – The First Serbian Uprising begins as an intro into Serbian Revolution. By 1817, Principality of Serbia proclaims self-rule from Ottoman Empire, thus becoming the first nation-state in Europe.
February 15 – New Jersey becomes the last northern state to abolish slavery.
February 16 – First Barbary War: Stephen Decatur leads a raid to burn the pirate-held frigate USS Philadelphia at Tripoli to deny her further use by the captors.
February 18 – Ohio University is chartered by the Ohio General Assembly.
February 21 – The Cornishman Richard Trevithick's newly built Penydarren steam locomotive operates on the Merthyr Tramroad between Penydarren in Merthyr Tydfil and Abercynon in South Wales, following several trials since February 13, the world's first locomotive to work on rails.
February – The 1804 Haiti Massacre, an ethnic cleansing, eradicates the white population on Haiti; it is not stopped until 22 April.
John Wedgwood founds the Royal Horticultural Society.
Thomas Charles is instrumental in founding the British and Foreign Bible Society.
March 10 – Louisiana Purchase: In St. Louis, a formal ceremony is conducted to transfer ownership of Louisiana Territory from France to the United States.
March 17 – First performance of Friedrich Schiller's play Wilhelm Tell, at Weimar under the direction of Johann Wolfgang von Goethe.
March 21 – The Napoleonic Code is adopted as French civil law.
April 2 – Forty merchantmen are wrecked when a convoy led by the HMS Apollo runs aground off Portugal.
April 4 – Samuel Taylor Coleridge aboard The Speedwell sails to the Mediterranean
April 5 – High Possil meteorite, the first recorded meteorite to fall in Scotland in modern times, falls at High Possil.
April 26 – Henry Addington resigns as Prime Minister of the United Kingdom.
May 10 – William Pitt the Younger begins his second term as Prime Minister of the United Kingdom.
May 14 – The Lewis and Clark Expedition departs from Camp Dubois and begins their historic journey by traveling up the Missouri River.
May 18 – Napoleon Bonaparte is proclaimed Emperor of the French by the French Senate.
May 21 – Père Lachaise Cemetery a 118 acres (0.48 km2) cemetery in Paris, France is opened.
June 9 – Beethoven's Symphony No. 3 in E–flat premiered in Vienna.
June 15 – The Twelfth Amendment to the U.S. Constitution is ratified by New Hampshire, and arguably becomes effective (subsequently vetoed by the Governor of New Hampshire).
July 11 – Alexander Hamilton is shot during a duel with Aaron Burr and dies the next day.
July 27 – The Twelfth Amendment to the U.S. Constitution ratified by Tennessee, removing doubt surrounding adoption.
August 20 – Lewis and Clark Expedition: The Corps of Discovery, whose purpose is to explore the Louisiana Purchase, suffers its only death when Sergeant Charles Floyd dies, apparently from acute appendicitis.
September 1 – German astronomer K. L. Harding discovers the asteroid Juno.
November 3 – The Treaty of St. Louis is signed by Quashquame and William Henry Harrison; controversy surrounding the treaty eventually causes the Sauk people to ally with the British during the War of 1812 and is the main cause of the Black Hawk War of 1832.
November 20 – Said bin Sultan, Sultan of Muscat and Oman starts to rule.
November 30 – The Democratic-Republican-controlled United States Senate begin an impeachment trial against Federalist-partisan Supreme Court of the United States Justice Samuel Chase on charges of political bias (he is acquitted by the United States Senate of all charges on March 1, 1805).
December 2 – Coronation of Napoleon I: At the cathedral of Notre Dame de Paris, Napoleon crowns himself as the first Emperor of the French in a thousand years. Witnessing this, Simón Bolívar dedicates himself to liberating Venezuela from Spanish rule.
December 3 – Thomas Jefferson defeats Charles C. Pinckney in the United States presidential election.
December 12 – Spain declares war on the United Kingdom.
The Nguyễn dynasty emperor Gia Long changes his country's official name from Đại Việt to Việt Nam.
Morphine is first isolated from the opium poppy by the German pharmacist, Friedrich Sertürner.
Matthew Flinders recommends that New Holland be renamed Australia (from the Latin "australis" meaning "of the south").
World population reaches 1 billion people.
January 1 – James Fannin, Texas revolutionary (d. 1836)
January 10 – Élie Frédéric Forey, Marshal of France (d. 1872)
January 20 – Eugène Sue, French novelist (d. 1857)
January 21 – Eliza Roxcy Snow, American poet (d. 1887)
February – James Bronterre O'Brien, Irish-born Chartist (d. 1864)
February 7 – John Deere, American industrialist (d. 1886)
February 13 – Claude-Étienne Minié, French army officer and weapon inventor (d. 1879)
March 8 – Alvan Clark, American telescope manufacturer (d. 1887)
March 14 – Johann Strauss Senior, Austrian composer (d. 1849)
March 17 – Jim Bridger, American trapper and explorer (d. 1881)
April 3 – Lucien Baudens, French military surgeon (d. 1857)
March 20 – Neal Dow, mayor of Portland and Father of Prohibition (d. 1897)
April 26 – Charles Goodyear, American politician (d. 1876)
May 16 – Elizabeth Peabody, Transcendental activist, educator (d. 1894)
Mikhail Glinka, Russian composer (d. 1857)
George Sand, French writer (d. 1876)
June 24 – Willard Richards, American religious leader (d. 1854)
July 4 – Nathaniel Hawthorne, American writer (d. 1864)
July 14 – Ludwig von Benedek, Austrian general (d. 1881)
July 20 – Richard Owen, English anatomist, paleontologist, and zoologist (d. 1892)
July 23 – Jane Irwin Harrison, de facto First Lady of the United States (d. 1846)
July 28 – Ludwig Feuerbach, German philosopher (d. 1872)
September 5 – William Alexander Graham, United States Senator from North Carolina (1840-1843), Confederate States Senator (1864-1865) (d. 1875)
September 8 – Eduard Mörike, German poet (d. 1875)
September 11 – Mercedes Marín del Solar, international Chilean poet and reform educator (d. 1866)
Louis Désiré Maigret, Roman Catholic bishop of Honolulu (d. 1882)
John Gould, English ornithologist (d. 1881)
October 18 – Mongkut, Rama IV, King of Siam (d. 1868)
October 24 – Wilhelm Eduard Weber, German physicist (d. 1891)
November 18 – Alfonso Ferrero La Marmora, Italian general and statesman (d. 1878)
November 23 – Franklin Pierce, 14th President of the United States (d. 1869)
December 7 – Noah Haynes Swayne, Associate Justice of the Supreme Court of the United States (d. 1884)
December 10 – Carl Gustav Jacob Jacobi, German mathematician (d. 1851)
December 13 – Joseph Howe, Canadian politician (d. 1873)
December 16 – Viktor Bunyakovsky, Ukrainian-Russian mathematician (d. 1889)
December 21 – Benjamin Disraeli, Prime Minister of the United Kingdom (d. 1881)
December 23 – Charles Augustin Sainte-Beuve, French literary critic (d. 1869)
Eugénie Luce, French educator (d. 1882)
James Mackay, Scottish-born New Zealand politician (d. 1875)
Andrew Nicholl, Northern Irish painter (d. 1886)
January 4 – Charlotte Ramsey Lennox, British author and poet (b. 1727)
January 15 – Dru Drury, English entomologist (b. 1725)
February 6 – Joseph Priestley, English chemist (b. 1733)
February 12 – Immanuel Kant, German philosopher (b. 1724)
March 3 – Giovanni Domenico Tiepolo, painter (b. 1727)
March 13 – Damodar Pande, Prime Minister of Nepal (b. 1752)
March 16 – Henrik Gabriel Porthan Finnish writer and historian (b. 1739)
March 21 – Louis Antoine, Duke of Enghien (executed) (b. 1772)
March 30 – Victor-François, 2nd duc de Broglie, Marshal of France (b. 1718)
April 9 – Jacques Necker, French statesman (b. 1732)
April 11 – Miklós Küzmics, Hungarian Slovenes writer, Catholic priest (b. 1737)
April 15 – Jean-Charles Pichegru, French general (strangled in prison) (b. 1761)
July 12 – Alexander Hamilton, American statesman (killed in a duel) (b. 1755)
September 4 – Richard Somers, American naval officer (killed in battle)
October 2 – Nicolas-Joseph Cugnot, French steam vehicle pioneer (b. 1725)
October 8 – Thomas Cochran (judge), Canadian judge (b. 1777)
November 5 – Maria Anna Adamberger, Austrian actress (b. 1752)
November 18 – Philip Schuyler, general in the American Revolution, a United States Senator from New York, father of Angelica Schuyler Church and Elizabeth Schuyler Hamilton (b. 1733)
November 23 – Richard Graves, English writer (b. 1715)
December 18 – Jacob ben Wolf Kranz, Lithuanian maggid (b. c. 1740)
1804 (MDCCCIV) was a leap year starting on Sunday (dominical letter AG) of the Gregorian calendar and a leap year starting on Friday (dominical letter CB) of the Julian calendar, the 1804th year of the Common Era (CE) and Anno Domini (AD) designations, the 804th year of the 2nd millennium, the 4th year of the 19th century, and the 5th year of the 1800s decade. As of the start of 1804, the Gregorian calendar was 12 days ahead of the Julian calendar, which remained in localized use until 1923.