|Centuries:||18th century – 19th century – 20th century|
|Decades:||1770s 1780s 1790s – 1800s – 1810s 1820s 1830s|
|Years:||1800 1801 1802 – 1803 – 1804 1805 1806|
- January 4 – William Symington demonstrates his Charlotte Dundas, the "first practical steamboat".
- January 30 – Monroe and Livingston sail for Paris to discuss, and possibly buy, New Orleans. They end completing the Louisiana Purchase.
- February 21 – Edward Despard and six others are hanged, drawn and quartered for plotting to assassinate king George III and to destroy the Bank of England
- February 24 – The Supreme Court of the United States, in Marbury v. Madison, establishes the principle of judicial review.
- March 1 – Ohio is admitted as the 17th U.S. state, retroactive from August 7, 1953.
- April 30 – Louisiana Purchase made by the United States from France.
- March 12 – Port Gibson, MS is chartered
- May 18 – The United Kingdom redeclares war on France after France refused to withdraw from Dutch territory.
- 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 of the world.
- August 3 – British begin Second Anglo-Maratha War against Sindhia of Gwalior
- September 20 – Irish rebel Robert Emmet is executed
- September 23 – The Battle of Assaye in India – British-lead troops defeat Maratha forces
- October 20 – Senate ratifies the Louisiana Purchase Treaty, doubling the size of the United States.
- November 30 – At the Cabildo building in New Orleans, Spanish representatives Governor Manuel de Salcedo and the Marqués de Casa Calvo, officially transfer Louisiana Territory to French representative Prefect Pierre Clément de Laussat (just 20 days later, France had transferred the same land to the United States as the Louisiana Purchase).
- Aargau, Graubünden, St. Gallen, Thurgau, the Ticino, Vaud become Swiss cantons.
- France – the Livre Tournois (Tours Pound) is replaced by the Franc.
- William Osgoode, Chief Justice of Lower Canada, rules that slavery is inconsistent with British Law.
- for more information, see Category:1803 births.
- for more information, see Category:1803 deaths.