1800 (MDCCC) was an exceptional common year starting on Wednesday in the Gregorian calendar and a leap year starting on Sunday in the Julian calendar. As of the start of 1800, the Gregorian calendar was 11 days ahead of the Julian calendar, which remained in localized use until 1923.
As of March 1 (O.S. February 18), when the Julian calendar acknowledged a leap day and the Gregorian calendar did not, the Julian calendar fell one day further behind, bringing the difference to 12 days until February 28 (O.S. February 16), 1900.
|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.