Events of 1820

• January 1 - Constitutionalist military insurrection at Cádiz leads to summoning of Spanish parliament (March 7) (See Mid-nineteenth century Spain.)
• January 28 - Russian expedition lead by Fabian Gottlieb von Bellingshausen and Mikhail Petrovich Lazarev approaches the Antarctic coast. (See History of Antarctica.)
• January 29 - George IV of the United Kingdom ascends the throne, ending the period known as the British Regency. There will be a gap of eleven years before the title Prince of Wales is next used.
• January 30 - Edward Bransfield lands on the Antarctic mainland. (See History of Antarctica.)
• February 6
  • Lord Thomas Cochrane occupies Valdivia in name of the republic of Chile.
  • 86 free African American colonists sail from New York City to Freetown, Sierra Leone.
• February 8 - William Tecumseh Sherman is born.
• February 20 - A revolt begins in Santa María Chiquimula, Totonicapán department of Guatemala.
• February 23 - The Cato Street conspiracy is exposed. The principals are executed on May 1.
• March 3 & 6 - Slavery in the United States: The Missouri Compromise becomes law.
• March 9 - King Ferdinand VII of Spain accepts the new constitution, beginning the Liberal Triennium ("Trienio Liberal").
• March 15 - Maine is admitted as the 23rd U.S. state.
• April
  • Hans Christian Ørsted discovers the relationship between electricity and magnetism.
  • Radical War is fought in Scotland.
• May 1 - Last hanging drawing and quartering in Britain – Cato Street conspirators for treason (only hanged and beheaded.)
• May 11 - Launch of HMS Beagle the ship that would later take young Charles Darwin on his scientific voyage.
• July - Constitutionalist revolution in the Kingdom of the Two Sicilies.
• July 20 - Saint Cronan's Boys National School opens in Bray, Co. Wicklow, Ireland under the title Bray Male School. It is the oldest school in Bray and its notable past pupils include former President of Ireland, Cearbhall Ó Dálaigh
• August 24 - Constitutionalist insurrection at Oporto, Portugal; revolution in Lisbon, September 15 (see Portugal's crises of the Nineteenth Century.).
• October 9 - Guayaquil declare independence from Spain. (See also History of Ecuador).
• October 25-November 20 - Congress of Troppau (Opava) between rulers of Russia, Austria and Prussia.
• December 3 - U.S. presidential election, 1820 - James Monroe is re-elected, virtually unopposed.

Undated