The legislative union of Great Britain and Ireland is completed under the Act of Union 1800, bringing about the United Kingdom of Great Britain and Ireland and abolition of the Parliament of Ireland.
Giuseppe Piazzi discovers the asteroid and dwarf planet Ceres.
First day of the 19th century.
January 3 – Toussaint Louverture triumphantly enters Santo Domingo, the capital of the former Spanish colony of Santo Domingo, which has become a colony of Napoleonic France.
January 31 – John Marshall is appointed Chief Justice of the United States.
February 4 – William Pitt the Younger resigns as Prime Minister of the United Kingdom.
February 9 – The Treaty of Lunéville ends the War of the Second Coalition between France and Austria. Under the terms of the treaty, Aachen is officially annexed by France.
February 17 – An electoral tie between Thomas Jefferson and Aaron Burr is resolved, when Jefferson is elected President of the United States and Burr Vice President by the United States House of Representatives.
February 27 – Washington, D.C. is placed under the jurisdiction of the United States Congress.
March 4 – Thomas Jefferson is sworn in as President of the United States.
March 10 – The first census is held in Great Britain. The population of England and Wales is determined to be 8.9 million, with London revealed to have 860,035 residents. 1.5 million people live in cities of 20,000 or more in England and Wales, accounting for 17% of the total English population.
March 14 – Henry Addington becomes First Lord of the Treasury and Chancellor of the Exchequer effectively Prime Minister of the United Kingdom.
March 21 – Second Battle of Abukir in Egypt: British troops defeat the French, but the British commander, Sir Ralph Abercromby, dies later of a wound received in the action.
March 23 – Tsar Paul I of Russia is murdered. He is succeeded by his son Alexander I.
April 2 – First Battle of Copenhagen (War of the Second Coalition): The Royal Navy of the United Kingdom under Admiral Sir Hyde Parker forces the Royal Dano-Norwegian Navy to accept an armistice. Vice-Admiral Horatio Nelson leads the main attack, deliberately disregarding his commander's signal to withdraw. He is created a Viscount on May 19; Denmark is forced to withdraw from the Second League of Armed Neutrality.
April 21 – Ranjit Singh is invested as Maharaja of Punjab.
May 10 – The pascha of Tripoli declares war on the United States by having the flagpole on the consulate chopped down.
June 7 – Portugal and Spain sign the Treaty of Badajoz; Portugal loses the city of Olivenza.
June 15 – A bull breaks through barriers at a bullfight in Madrid, killing two people (including the mayor of Torrejón de Ardoz) and injuring a number of other spectators.
June 27 – Cairo falls to British troops.
July 6 – Battle of Algeciras: The French fleet defeats the British fleet.
July 7 – Toussaint Louverture promulgates a reforming constitution for Santo Domingo, declaring himself emperor for life of the entire island of Hispaniola and nominally abolishing slavery.
July 12 – Second Battle of Algeciras: The British fleet defeats the French and Spanish fleets.
July 18 – Napoleon signs a Concordat with Pope Pius VII.
August 1 – Action of 1 August 1801 (First Barbary War): United States Navy schooner USS Enterprise (1799) captures the 14-gun Tripolitan corsair polacca Tripoli off the north African coast in a single-ship action.
September 30 – The Treaty of London is signed for preliminary peace between the French First Republic and the United Kingdom of Great Britain and Ireland.
October 17 – A coup d'état is staged in the Batavian Republic.
November 16 – The first edition of New-York Evening Post is printed.
December 15 – Hadži Mustafa Pasha, Ottoman commander and politician, is assassinated in Belgrade, Sanjak of Smederevo by Kučuk-Alija.
The first census is held in France.
Joseph Marie Jacquard develops a loom where the pattern being woven is controlled by punched cards.
Philippe Pinel publishes Traité médico-philosophique sur l'aliénation mentale; ou la manie, presenting his enlightened humane psychological approach to the management of psychiatric hospitals. Translated into English by D. D. Davis as Treatise on Insanity in 1806, it is influential on both sides of the Atlantic during the nineteenth century.
Ultraviolet radiation is discovered by Johann Wilhelm Ritter.
The magnum opus Disquisitiones Arithmeticae of Carl Friedrich Gauss is published.
Supreme Council, Scottish Rite (Southern Jurisdiction, USA) is founded within Freemasonry.
January 3 – Gijsbert Haan, Dutch-American religious leader (d. 1874)
January 11 – Honório Hermeto Carneiro Leão, Marquis of Paraná, Brazilian politician (d. 1856)
January 14 – Jane Welsh Carlyle, wife of essayist Thomas Carlyle (d. 1866)
Jean-Baptiste Boussingault, French chemist (d. 1887)
Thomas Cole, American artist (d. 1848)
February 13 – János Kardos, Hungarian Slovenes evangelical priest, teacher and writer (d. 1875)
February 21 – John Henry Newman, English Roman Catholic Cardinal (d. 1890)
May 5 – Pío Pico, last Governor of Alta California (d. 1894)
May 9 – Peter Hesketh-Fleetwood, English Member of Parliament and developer (d. 1866)
May 11 – Henri Labrouste, French architect (d. 1875)
May 16 – William H. Seward, 24th United States Secretary of State (d. 1872)
May 17 – Lovisa Åhrberg, first woman doctor and surgeon in Sweden (d. 1881)
May 31 – Johann Georg Baiter, Swiss philogist and textual critic (d. 1877)
June 1 – Brigham Young, American religious leader and colonizer (d. 1877)
June 4 – James Pennethorne, English architect (d. 1871)
June 14 – Heber C. Kimball, American religious leader (d. 1868)
June 30 – Frédéric Bastiat, French philosopher (d. 1850)
July 5 – David Farragut, American admiral (d. 1870)
July 14 – Johannes Peter Müller, German physiologist, comparative anatomist, ichthyologist, and herpetologist (d. 1858)
July 27 – George Biddell Airy, English mathematician and astronomer (d. 1892)
July 29 – George Bradshaw, English publisher (d. 1853)
September 1 – Hortense Allart, French writer (d. 1879)
September 3 – Christian Erich Hermann von Meyer, German palaeontologist (d. 1869)
– Friedrich Frey-Herosé, member of the Swiss Federal Council (d. 1873)
– Carl August von Steinheil, German engineer and astronomer (d. 1870)
October 23 – Albert Lortzing, German composer (d. 1851)
Karl Baedeker, German author and publisher (d. 1859)
Vincenzo Bellini, Italian composer (d. 1835)
November 10 – Vladimir Dal, Russian lexicographer (d. 1872)
November 13 – Queen Elisabeth Ludovika of Bavaria, queen of Prussia (d. 1873)
December 11 – Christian Dietrich Grabbe, German writer (d. 1836)
Dai Xi, Chinese painter (d. 1860)
Franciszek Ksawery Godebski, Polish writer (d. 1869)
Thierry Hermès, German-born French businessman and founder of Hermès (d. 1878)
Brita Sofia Hesselius, Swedish photographer (d. 1866)
Julius Plücker, German mathematician and physicist (d. 1868)
January 2 – Johann Kaspar Lavater, Swiss physiognomist (b. 1741)
February 7 – Daniel Chodowiecki, Polish painter (b. 1726)
March 19 – Ambrosio O'Higgins, 1st Marquis of Osorno, Spanish viceroy of Peru and Governor of the Captaincy General of Chile, father of Bernardo O'Higgins
March 21 – Andrea Luchesi, Italian composer (b. 1741)
March 23 – Tsar Paul I of Russia (b. 1754)
March 25 – Novalis, German poet (b. 1772)
March 28 – Ralph Abercromby, British general (b. 1734)
April 2 – Thomas Dadford, Jr., British engineer
April 7 – Noël François de Wailly, French lexicographer (b. 1724)
May 17 – William Heberden, English physician (b. 1710)
June 4 – Frederick Muhlenberg, first Speaker of the U.S. House of Representatives (b. 1750)
June 14 – Benedict Arnold, American Revolution hero and traitor (b. 1741)
September 19 – Johann Gottfried Koehler, German astronomer (b. 1745)
October 3 – Philippe Henri, marquis de Ségur, Marshal of France (b. 1724)
November 4 – William Shippen, American physician and Continental Congressman (b. 1712)
November 5 – Motoori Norinaga, Japanese philologist and scholar (b. 1730)
November 24 – Franz Moritz von Lacy, Austrian field marshal (b. 1725)
Date unknown – Ulrica Arfvidsson, Swedish fortune teller (b. 1734)
1801 (MDCCCI) 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 1801st year of the Common Era (CE) and Anno Domini (AD) designations, the 801st year of the 2nd millennium, the 1st year of the 19th century, and the 2nd year of the 1800s decade. As of the start of 1801, the Gregorian calendar was 12 days ahead of the Julian calendar, which remained in localized use until 1923.