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1809 (MDCCCIX)was a common year starting on Sunday of the Gregorian calendar and a common year starting on Friday of the Julian calendar, the 1809th year of the Common Era (CE) and Anno Domini (AD) designations, the 809th year of the 2nd millennium, the 9th year of the 19th century, and the 10th and last year of the 1800s decade. As of the start of 1809, the Gregorian calendar was 12 days ahead of the Julian calendar, which remained in localized use until 1923.
- April 9 – Tiroleans rise, under the command of Andreas Hofer, against French and Bavarian occupation.
- April 10 – Napoleonic Wars – The War of the Fifth Coalition begins when forces of the Austrian Empire invade Bavaria.
- April 11–15 – Napoleonic Wars – Battle of the Basque Roads: The British Royal Navy defeats the French fleet in the mouth of the Charente, although officers on both sides face subsequent courts-martial.
- April 14 – Battle of Abensberg, Bavaria: Napoleon defeats Austria.
- April 18 – The 2,000 Guineas Stakes horse race is first run in England.
- April 19 – War of the Fifth Coalition –
- April 22 – Battle of Eckmühl: French troops under Napoleon I and Marshal Davout defeat the Austrians, under Archduke Charles.
- May 5
- May 10 – Gustav IV Adolf is officially deposed from the Swedish throne, by the Riksdag of the Estates.
- May 10–11 – Peninsular War – Battle of Grijó: the Anglo-Portuguese Army, commanded by Sir Arthur Wellesley, defeats the French army, commanded by Marshal Soult, in Portugal.
- May 12 – Peninsular War – Second Battle of Porto: The Anglo-Portuguese Army, commanded by Wellesley, drives the French army, commanded by Marshal Soult, out of Porto, and forces them to retreat from the country.
- May 17 – Napoleon I of France orders the annexation of the Papal States to the French Empire. When he announces that the Pope's secular power has ended, the Pope excommunicates him.
- May 21 – Battle of Aspern-Essling: Austrian troops under Archduke Karl beat the French under Napoleon, in a hard-fought battle.
- May 24 – Dartmoor Prison opens in England, to house French prisoners of war.
- May 31 – Mauritius campaign of 1809–11 – Action of 31 May 1809 in the Bay of Bengal: The French frigate Caroline, operating from Isle de France (Mauritius), captures most of a British East India Company fleet.
- June 6 – Sweden promulgates a new Instrument of Government, which restores political power to the Riksdag of the Estates, after authoritarian rule since 1772. On the same day, Duke Charles (uncle of the deposed king Gustav IV Adolf) is elected King, under the name Charles XIII.
- June 7 – Shoja Shah of the Durrani Empire signs a treaty with the British; only weeks later, he is succeeded by Mahmud Shah.
- June 14 – A French victory, in the battle of Raab, prevents archduke John of Austria from bringing any significant force to the battle of Wagram.
- October 8 – Prince Klemens von Metternich becomes foreign minister of the Austrian Empire.
- October 11 – Along the Natchez Trace in Tennessee, explorer Meriwether Lewis dies under mysterious circumstances, at an inn called Grinder's Stand.
- October 14 – The Treaty of Schönbrunn cedes the Illyrian Provinces to France.
- November 18 – Napoleonic Wars: Action of 18 November 1809 – In the Bay of Bengal, a French frigate squadron captures three East Indiamen mainly carrying recruits for the presidency armies.
- November 19 – Battle of Ocaña: A Spanish army is ridden down, and 4,000 are killed and wounded by French forces.
- November 25 – Benjamin Bathurst, a British diplomat, mysteriously disappears (possibly murdered) in Perleberg, west of Berlin.
- December – Boyd massacre: Whangaroa Māori people kill and eat 66 crew and passengers of the brigantine Boyd in New Zealand.
- December 25 – Dr. Ephraim McDowell performs the first ovariotomy, removing a 22-pound tumor.
- December 26 – A British invasion force leaves Vlissingen.
- December 30 – Wearing masks at balls is forbidden in Boston, Massachusetts.