|Centuries:||17th century – 18th century – 19th century|
|Decades:||1770s 1780s 1790s – 1800s – 1810s 1820s 1830s|
|Years:||1797 1798 1799 – 1800 – 1801 1802 1803|
- World population hits 1 billion.
- March 14 – Cardinal Barnaba Chiaramonti is elected pope as Pius VII.
- March 21 – Pius VII is ordained.
- April 24 – U.S. Library of Congress founded.
- May 5 – Great Britain passes the Act of Union to join Great Britain and Ireland into the United Kingdom of Great Britain and Ireland to take effect on 1 January 1801. The act is signed by King George III in August.
- May 15 – Napoleon Bonaparte crosses the Alps and invades Italy.
- June 14 – Battle of Marengo, Napoleon defeats the Austrian troops near Marengo, Italy.
- June 2 – First smallpox vaccination in North America, at Trinity, Newfoundland.
- June 27 – Pascha Jussuf Karamanli of Tripoli declares war on Sweden by having the flagpole on the consulate chopped down.
- September 5 – The island of Malta, that was occupied by the French, is conquered by British troops.
- November 1 – U.S. President John Adams becomes the first President of the United States to live in the Executive Mansion (later renamed the White House).
- November 17 – The U.S. Congress holds its first Washington, DC session.
- December 3 – Battle of Hohenlinden, the French army defeats the Austrian troops.
- December 24 – An assault on Napoleon Bonaparte fails in Paris.
- December 24 – Pierre Coudrin and Henriette Aymer de la Chevalerie found the Congregation of the Sacred Hearts of Jesus and Mary in Paris.
- Invention of the voltaic pile by Alessandro Volta: the first chemical battery
- The infrared radiation is discovered by Wilhelm Herschel.
- The Althing of Iceland, the world's oldest parliament, is abolished.
- for more information, see Category:1800 births.
- for more information, see Category:1800 deaths.