|Covid-19|January 1 – The first edition of Alexandre Balthazar Laurent Grimod de La Reynière's Almanach des gourmands, the first guide to restaurant cooking, is published in Paris.
January 5 – William Symington demonstrates his Charlotte Dundas, the "first practical steamboat", in Scotland.
January 30 – Monroe and Livingston sail for Paris to discuss, and possibly buy, New Orleans; they end up completing the Louisiana Purchase.
February 19 – Act of Mediation issued by Napoleon Bonaparte establishes the Swiss Confederation to replace the Helvetic Republic. Under the terms of the act, Graubünden, St. Gallen, Thurgau, the Ticino and Vaud become Swiss cantons.
February 20 – Kandy took by a British detachment.
February 21 – Edward Despard and six others are hanged and beheaded for plotting to assassinate King George III of the United Kingdom and to destroy the Bank of England.
February 24 – Marbury v. Madison: The Supreme Court of the United States establishes the principle of judicial review.
February 25 – A major redistribution of territorial sovereignty within the Holy Roman Empire is enacted via an act known as the Reichsdeputationshauptschluss.
March 1 – Ohio is admitted as the 17th U.S. state, retroactive from August 7, 1953.
March 9 – Aargau becomes a Swiss canton.
March–April – The franc germinal is introduced in France.
April 26 – The L'Aigle meteorite falls in Normandy, causing a shift in scientific opinion on the origin of meteorites.
April 30 – Louisiana Purchase is made by the United States from France.
May – The First Consul of France Citizen Bonaparte begins making preparations to invade England.
May 18 – The United Kingdom resumes war on France after France refuses to withdraw from Dutch territory which begins the Napoleonic Wars.
May 19 – Master Malati, a Coptic Christian leader, is beheaded by a Muslim mob in Cairo, Egypt.
July 4 – The Louisiana Purchase is announced to the American people.
July 5 – The convention of Artlenburg leads to the French occupation of Hanover (which had been ruled by the British king).
July 23 – Robert Emmet's uprising in Ireland begins.
July 26 – The wagonway between Wandsworth and Croydon is opened, being the first public railway line in England.
August 3 – The British begin the Second Anglo-Maratha War against the Scindia of Gwalior.
September 11 – Battle of Delhi, during the Second Anglo-Maratha War, between British troops under General Lake, and Marathas of Scindia's army under General Louis Bourquin.
September 20 – Irish rebel Robert Emmet is executed.
September 23 – Battle of Assaye (India): British-led troops defeat Maratha forces.
October 20 – The Senate ratifies the Louisiana Purchase Treaty, doubling the size of the United States.
November 18 – Battle of Vertières: The Haitian army led by Jean-Jacques Dessalines defeats the army of Napoleon.
November 30 – At the Cabildo in New Orleans, Spanish representatives Governor Manuel de Salcedo and the Marqués de Casa Calvo officially transfer Louisiana (New Spain) to French representative Prefect Pierre Clément de Laussat. Barely three weeks later, in the same location on December 20, France transfers the same land to the United States as the Louisiana Purchase, with William C. C. Claiborne and James Wilkinson officiating for the U.S.
Thomas Bruce, 7th Earl of Elgin begins removal of the Elgin Marbles from the Parthenon in Athens where they are at risk of destruction during the Ottoman occupation of Greece.
Odisha, India is occupied by the British under the British East India Company after the Second Anglo-Maratha War.
Major breakthroughs in chemistry occur, with the identification of the elements rhodium, palladium, osmium, iridium, and cerium.
February 2 – Albert Sidney Johnston, American Confederate general (d. 1862)
John Sutter, German-American pioneer (d. 1880)
Karl Friedrich Schimper, German botanist, naturalist and poet (d. 1867)
February 26 – Arnold Adolph Berthold, German physiologist and zoologist (d. 1861)
March 12 – Guillaume de Felice, Savoy nobleman and abolitionist (d. 1871)
March 16 – Nikolay Yazykov, Russian poet and Slavophile (d. 1846)
April 7 – Flora Tristan, French feminist (d. 1844)
Jeremiah E. Cary, American politician (d. 1888)
Albrecht von Roon, Prime Minister of Prussia (d. 1879)
May 12 – Justus von Liebig, German chemist (d. 1873)
May 24 – Charles Lucien Bonaparte, French naturalist and ornithologist (d. 1857)
Edward Bulwer-Lytton, English novelist and playwright (d. 1873)
Ralph Waldo Emerson, American writer (d. 1882)
June 8 – Amalia Assur, Swedish dentist (d. 1889)
June 24 – George James Webb, English-born composer (d. 1887)
July 10 – William Todd (1803–1873), American businessman, Canadian senate nominee
July 20 – John Hymers, English mathematician (d. 1887)
July 24 – Adolphe Adam, French composer (d. 1856)
July 31 – John Ericsson, Swedish inventor and engineer (d. 1889)
August 3 – Sir Joseph Paxton, English gardener, architect and Member of Parliament (d. 1865)
August 13 – Vladimir Odoyevsky, Russian philosopher, writer, music critic (d. 1869)
August 18 – Nathan Clifford, American politician and Associate Justice of the Supreme Court of the United States (d. 1881)
Jan Erazim Vocel, Czech poet, archaeologist, historian and cultural revivalist (d. 1871)
Gustaf Wappers, Belgian painter (d. 1874)
August 27 – Edward Beecher, American theologian (d. 1895)
September 4 – Sarah Childress Polk, First Lady of the United States (d. 1891)
September 11 – Francisca Zubiaga y Bernales, first lady of Peru and controversial socialite (d. 1835)
September 27 – Samuel Francis Du Pont, American admiral (d. 1865)
September 28 – Prosper Mérimée, French writer (d. 1870)
September 29 – Mercator Cooper, American sea captain (d. 1872)
October 5 – Friedrich Bernhard Westphal, Danish-German painter (d. 1844)
October 16 – Robert Stephenson, English civil engineer (d. 1859)
November 14 – Jacob Abbott, American writer (d. 1879)
Christian Doppler, Austrian mathematician (d. 1853)
Gottfried Semper, German architect (d. 1879)
December 5 – Fyodor Tyutchev, great Russian Romantic poet (d. 1873)
December 6 – Susanna Moodie, English writer (d. 1885)
December 11 – Hector Berlioz, French composer (d. 1869)
Charles Lafontaine, Swiss mesmerist (d. 1892)
January 18 – Ippolit Bogdanovich, Russian poet (b. 1743)
January 23 – Arthur Guinness, Irish brewer (b. 1725)
February 3 – María Isidra de Guzmán y de la Cerda, Spanish scholar (b. 1768)
February 9 – Jean François de Saint-Lambert, French poet (b. 1716)
February 18 – Johann Wilhelm Ludwig Gleim, German poet (b. 1719)
February 20 – Marie Dumesnil, French actress (b. 1713)
February 23 – Praskovia Kovalyova-Zhemchugova, Russian serf actress and opera soprano (b. 1768)
February 21 – Edward Despard, British revolutionary (b. 1751)
March 14 – Friedrich Gottlieb Klopstock, German poet (b. 1724)
April 2 – Sir James Montgomery, 1st Baronet, Scottish politician and judge (b. 1721)
April 7 – Toussaint L'Ouverture, Haitian revolutionary (b. 1743)
June 24 – Matthew Thornton, American signer of the Declaration of Independence (b. 1714)
August 24 – James Napper Tandy, Irish republican
September 5 – Pierre Choderlos de Laclos, French general and author (b. 1741)
September 13 – John Barry, officer in the Continental Navy during the American Revolutionary War and later in the United States Navy (b. 1745)
Gian Francesco Albani, Italian Catholic cardinal (b. 1719)
François Devienne, French composer (b. 1759)
September 17 – Franz Xaver Süssmayr, Austrian composer (b. 1766)
October 2 – Samuel Adams, American revolutionary leader (b. 1722)
October 8 – Vittorio Alfieri, Italian dramatist and poet (b. 1749)
October 14 – Louis Claude de Saint-Martin, French philosopher (b. 1743)
October 26 – Granville Leveson-Gower, 1st Marquess of Stafford, English politician (b. 1721)
October 31 – Pandara Vanniyan the last King of Vanni defeated by Lt. von Driberg
November 18 – Ditlevine Feddersen, Norwegian culture figure (b. 1727)
December 18 – Johann Gottfried Herder, German philosopher and writer (b. 1744)
December 30 – Francis Lewis, American signer of the Declaration of Independence (b. 1713)
Jacques-Donatien Le Ray de Chaumont, French Father of the American Revolution (b. 1726)
Moscho Tzavela, Greek-Souliote heroine (b. 1760)
1803 (MDCCCIII) was a common year starting on Saturday (dominical letter B) of the Gregorian calendar and a common year starting on Thursday (dominical letter D) of the Julian calendar, the 1803rd year of the Common Era (CE) and Anno Domini (AD) designations, the 803rd year of the 2nd millennium, the 3rd year of the 19th century, and the 4th year of the 1800s decade. As of the start of 1803, the Gregorian calendar was 12 days ahead of the Julian calendar, which remained in localized use until 1923.