1803 was a common year starting on Saturday (see link for calendar).
- January 30 - Monroe and Livingston sail for Paris to discuss, and possibly buy, New Orleans. They end completing the Louisiana Purchase.
- 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.
- 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).
- February 2 - Albert Sidney Johnston, Confederate general (d. 1862)
- February 15 - John Sutter, California pioneer (d. 1880)
- April 7 - Flora Tristan, French feminist (d. 1844)
- May 12 - Justus von Liebig, German chemist (d. 1873)
- May 24 - Charles Lucien Bonaparte, French naturalist and ornithologist (d. 1857)
- May 25 - Ralph Waldo Emerson, American writer (d. 1882)
- May 25 - Edward George Bulwer-Lytton, English novelist and playwright (d. 1873)
- June 24 - George James Webb, English-born composer (d. 1887)
- July 24 - Adolphe Charles Adam, French composer (d. 1856)
- July 31 - John Ericsson, Swedish inventor and engineer (d. 1889)
- September 4 - Sarah Childress Polk, First Lady of the United States (d. 1891)
- September 27 - Samuel Francis du Pont, American admiral (d. 1865)
- September 28 - Prosper Mérimée, French writer (d. 1870)
- November 14 - Jacob Abbott, American writer (d. 1879)
- November 29 - Christian Doppler, Austrian mathematician (d. 1853)
- December 11 - Hector Berlioz, French composer (d. 1869)
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