1818 (MDCCCXVIII) was a common year starting on Thursday (dominical letter D) of the Gregorian calendar and a common year starting on Tuesday (dominical letter F) of the Julian calendar, the 1818th year of the Common Era (CE) and Anno Domini (AD) designations, the 818th year of the 2nd millennium, the eighteenth year of the 19th century, and the ninth year of the 1810s decade. Note that the Julian day for 1818 is 12 calendar days difference, which continued to be used from 1582 until the complete conversion of the Gregorian calendar was entirely done in 1929.
- January 1
- January 3 (21:52 UTC) – Venus occults Jupiter. It is the last occultation of one planet by another before November 22, 2065.
- January 6 – Treaty of Mandeswar brings an end to the Third Anglo-Maratha War, ending the dominance of Marathas, and enhancing the power of the British East India Company, which controls territory occupied by 180 million Indians.
- January 11 – Percy Bysshe Shelley's "Ozymandias" is published pseudonymously in London.
- February 5 – Upon his death, King Charles XIII of Sweden (Charles II of Norway) is succeeded on both thrones by his adoptive son Charles XIV/III John, starting the Royal House of Bernadotte.
- February 12 – Chile proclaims its independence from Spain.
- March 11 – Mary Shelley's Frankenstein is published anonymously in London.
- March 15 – First Seminole War: General Andrew Jackson and his American army invade Florida.
- March 22 – Easter Day in Western Christianity falls on its earliest possible date. In Western Christianity, it won't occur on this date again until 2285.
- April – John Ross sets sail in search of the Northwest Passage.
- April 1 – First Seminole War: Battle of Miccosukee, Florida: General Andrew Jackson defeats chief Kinhagee.
- April 4 – The United States Congress adopts the flag of the United States as having thirteen red and white stripes and one star for each state (twenty) with additional stars to be added whenever a new state is added to the Union.
- April 5 – Battle of Maipú in the Chilean War of Independence: Patriot rebels led by José de San Martín decisively defeat the Spanish Royalists.
- April 7 – Brooks Brothers, the oldest men's clothier in the United States, opens its first store on the northeast corner of Catherine and Cherry Streets in New York City, where the later South Street Seaport stands.
- April 14–August 9 – United States Survey of the Coast operations suspended.
- May 11 – Charles XIV of Sweden–Norway is crowned king of Sweden.
- The Westmorland Gazette is first published at Kendal in the Lake District of England; in July, Thomas De Quincey will begin a 16-month term as editor.
- August 1 – Separate Topographical Bureau of the United States Department of War established.
- September – Sir Stamford Raffles sets out to visit Lord Hastings, the Governor-General of India, to gain his approval to establish a trading station at the southern tip of the Malay Peninsula (modern-day Singapore).
- September 7 – Carl III of Sweden–Norway is crowned king of Norway, in Trondheim.
- September 23 – Border markers are formally installed for the European territory of Moresnet.
- October 5 – Claudine Thévenet (known as 'Mary of St. Ignatius') founds the Roman Catholic order Religieuses de Jésus-Marie ("Religious of Jesus And Mary") in Lyon, France.
- October 20 – Treaty between the U.S. and the United Kingdom establishes the boundary between the U.S. and British North America as the 49th parallel from the Lake of the Woods to the Rocky Mountains, additionally creating the Northwest Angle.
- November 11 – Anglo-Chinese College is founded by Robert Morrison in Malacca (later renamed Ying Wa College).
- November 16 – The Saint Louis Academy, which later becomes Saint Louis University, is founded by Reverend Louis William Valentine Dubourg.
- December 3 – Illinois is admitted as the twenty-first U.S. state.
- December 13 – Cyril VI of Constantinople quits his place as an Ecumenical Pariarch.
- December 24 – The Christmas carol "Silent Night" (Stille Nacht) with words by the priest Josef Mohr set to music by organist Franz Xaver Gruber is first performed at St. Nikolaus parish church in Oberndorf bei Salzburg in Austria.
- The Afghans defeat a Persian invasion at the Battle of Kafir Qala.
- The Old Vic theatre founded (as the Royal Coburg Hall) in London.
- The first edition of the Farmers' Almanac is published in the United States.
- First Serbian dictionary, made by Vuk Karadžić.
- Besses o' th' Barn brass band is formed in Whitefield, near Manchester, by this date.
- January 30 – Artúr Görgey, Hungarian military general and politician (d. 1916)
- February 14 – Frederick Douglass (his day of birth was never established; he adopted this date), American abolitionist author and statesman (d. 1895)
- February 18 – Pedro Figueredo, Cuban poet, musician, and freedom writer (d. 1870)
- March 11 – Henri Étienne Sainte-Claire Deville, French chemist (d. 1881)
- March 15 – Mariano Álvarez, Filipino general (d. 1924)
- March 22 – John Ainsworth Horrocks, English-born explorer of South Australia (d. 1846)
- March 24 – William E. Le Roy, American admiral (d. 1888)
- April 4 – Thomas Mayne Reid, Irish-American novelist (d. 1883)
- April 8 –
- April 17 – Emperor Alexander II of Russia (d. 1881)
- April 19 – Sir Arthur Elton, seventh Baronet, writer and Liberal Party politician (d. 1883)
- May 5 – Karl Marx, German political philosopher (d. 1883)
- June 17
- June 18 – Angelo Secchi, Italian astronomer (d. 1878)
- July 1 – Ignaz Semmelweis, Hungarian physician and obstetrician (d. 1865)
- July 18 – Celadon Leeds Daboll, American merchant and inventor (d. 1866)
- July 22 – J. Gregory Smith, Vermont governor (d. 1891)
- July 27 – Agostino Roscelli, Italian priest and founder of the Institute of Sisters of the Immaculata (d. 1902)
- July 30 – Emily Brontë, British novelist (d. 1848)
- September 1 – José María Castro Madriz, first President of Costa Rica and founder of the republic (d. 1892)
- September 12 – Richard Jordan Gatling, American inventor and gunsmith (d. 1903)
- September 27 – Adolph Wilhelm Hermann Kolbe, German chemist (d. 1884)
- October 8 – John Henninger Reagan, American Confederate politician (d. 1905)
- October 15 – Irvin McDowell, American general (d. 1885)
- October 18 – Edward Ord, U.S. Army officer (d. 1883)
- November 5 – Benjamin Franklin Butler, American lawyer, politician, and general (d. 1893)
- November 9 (October 28 (O.S.)) – Ivan Turgenev, Russian writer (d. 1883)
- November 29 – George Brown, Canadian politician (d. 1880)
- December 13 – Mary Todd Lincoln, First Lady of the United States (d. 1882)
- December 24 – James Prescott Joule, English physicist (d. 1889)
- January 11 – Johann David Wyss, Swiss author (b. 1743)
- February 5 – Charles XIII/Charles II, King of Sweden and Norway (b. 1748)
- February 13 – George Rogers Clark, American Revolutionary leader (b. 1752)
- February 15 – Friedrich Ludwig, Fürst zu Hohenlohe-Ingelfingen, Prussian general (b. 1746)
- May 2 – Herman Willem Daendels, Governor-General of the Dutch East Indies (b. 1762)
- May 10 – Paul Revere, American patriot and silversmith (b. 1735)
- May 26 – Michael Andreas Barclay de Tolly, Russian military commander (b. 1761)
- June 12 – Egwale Seyon, Emperor of Ethiopia
- August 12 – Nikolay Novikov, Russian writer (b. 1744)
- August 22 – Warren Hastings, English Governor-General of India (b. 1732)
- August 24 – James Carr (Massachusetts politician), U.S. Congressman (b. 1777)
- September 1 – Robert Calder, British naval officer (b. 1745)
- October 5 – Nancy Hanks, mom of Abraham Lincoln (b. 1784)
- October 28 – Abigail Adams, First Lady of the United States (b. 1744)
- October 28 – Henri Jacques Guillaume Clarke, duc de Feltre, French marshal and politician (b. 1765)
- November 17 – Charlotte of Mecklenburg-Strelitz, queen of George III of the United Kingdom (b. 1744)
- December 25 – Catherine-Dominique de Pérignon, Marshal of France (b. 1754)