|Covid-19|January 4 – King Ferdinand I of the Two Sicilies is succeeded by his son Francis I of the Two Sicilies.
February 3 – Vendsyssel-Thy, once part of the Jutland peninsula that formed westernmost Denmark, becomes an island after a flood drowns its 1 km wide isthmus.
February 9 – After no presidential candidate receives a majority of electoral votes, the United States House of Representatives elects John Quincy Adams President of the United States.
February 10 – Simón Bolívar gives up his title of dictator of Peru and takes the alternative title of El Libertador.
February 12 – Second Treaty of Indian Springs: The Creek cede the last of their lands in Georgia to the United States government, and migrate west.
March 1 – The outbound British East Indiaman Kent (1820) is destroyed by fire in the Bay of Biscay with the loss of more than 80 lives, but over 550 are saved by passing ships.
March 2 – Capture of the Anne: Roberto Cofresí, one of the last successful pirates in the Caribbean, is defeated by an international naval force.
March 4 – John Quincy Adams is sworn in as President of the United States.
March 17 – Founding of insurance company The Norfolk & Dedham Group.
April 17 – Charles X of France recognizes Haiti, 21 years after it expelled the French following the successful Haitian Revolution, and demands the payment of 150 million gold francs, 30 million of which Haiti must finance through France itself, as down payment.
May 26 – Two Unitarian Christian bodies, the American Unitarian Association in the United States and the British and Foreign Unitarian Association in the United Kingdom are founded, coincidentally on the same date.
July 6 – The Duke of Schleswig-Holstein-Sonderburg-Beck gains possession of Glücksburg and changes his title to Friedrich Wilhelm, Duke of Schleswig-Holstein-Sonderburg-Glücksburg. The line of Schleswig-Holstein-Sonderburg-Glücksburg later becomes the royal house of Greece, Denmark and Norway.
July 18 – Uruguay secedes from Brazil.
July 30 – Malden Island (an uninhabited island in the central Pacific Ocean) is discovered by George Byron, 7th Baron Byron.
August 6 – Bolivia gains its independence from Spain as a republic with the instigation of Simón Bolívar.
August 18 – Scottish adventurer Gregor MacGregor issues a £300,000 loan with 2.5% interest through the London bank of Thomas Jenkins & Company for the fictitious Central American republic of Poyais. His actions lead to the Panic of 1825, the first modern stock market crash, in England.
August 25 – Uruguay is declared independent of the Empire of Brazil by the Thirty-Three Orientals, a militant revolutionary group led by Juan Antonio Lavalleja.
September – The Lady Margaret Boat Club is founded by 12 members of St John's College, Cambridge.
September 25 – General Hendrik Merkus de Kock lifts the siege of Jogjakarta, the first major action of the Java War.
September 27 – The world's first modern railway, the Stockton and Darlington Railway, opens in England.
October 7 – The Miramichi Fire, a forest fire, breaks out in New Brunswick (Canada).
October 21 – PS Comet II sinks off Gourock (Scotland) with the loss of 62 lives.
October 26 – The Erie Canal opens, providing passage from Albany, New York to Buffalo and Lake Erie.
December 1 (November 19 O.S.) – Nicholas I of Russia succeeds his older brother Alexander I.
December 26 (December 14 O.S.) – Some Imperial Russian Army officers stage the Decembrist Revolt against Nicholas's accession in Saint Petersburg, but it is thoroughly suppressed by the government.
The first horse-drawn omnibuses established in London.
Hans Christian Ørsted reduces aluminium chloride to produce metallic aluminium.
History of Brisbane: The Australian city of Brisbane is founded.
A new Combination Act in the United Kingdom makes trade unions legal according to narrowly defined principles.
Artisans and skilled mechanics receive a right to leave the United Kingdom.
Minh Mạng outlaws the teaching of Christianity in Vietnam.
The United States Postal Service starts a dead letter office.
Estimation: London becomes the largest city in the world, taking the lead from Beijing.
January 25 – George Pickett, American Confederate general (d. 1876)
January 31 – Miska Magyarics, Slovene poet in Hungary (d. 1883)
February 8 – Henri Giffard, French engineer and pioneer in airship technology (d. 1882)
March 13 – Hans Gude, Norwegian romanticist landscape painter (d. 1903)
March 21 – Alexander Mozhaysky, Russian aeronautical pioneer (d. 1890)
March 22 – Jane Sym, second wife of Canada's second prime minister (d. 1893)
April 24 – Robert Michael Ballantyne, Scottish novelist (d. 1894)
May 4 – Sir Thomas Henry Huxley, English biologist (d. 1895)
May 8 – George Bruce Malleson, English officer and author (d. 1898)
May 9 – George Davidson, English-born geodesist, astronomer, geographer, surveyor, and engineer in the United States (d. 1911)
June 3 – Sophie Sager, Swedish women's rights activist (d. 1902)
July 21 – Práxedes Mateo Sagasta, Spanish politician, eight-time Prime Minister of Spain (d. 1903)
August 31 – Robert Dunsmuir Scottish industrialist and politician (d. 1889)
September 11 – Eduard Hanslick, Austrian music critic (d. 1904)
September 13 – William Henry Rinehart, American sculptor (d. 1874)
September 17 – Lucius Quintus Cincinnatus Lamar II, American politician and Associate Justice of the Supreme Court of the United States (d. 1893)
September 25 – Joachim Heer, Swiss politician (d. 1879)
October 8 – Paschal Beverly Randolph, American occultist (d. 1875)
October 10 – Paul Kruger, Boer resistance leader (d. 1904)
October 13 – Charles Frederick Worth, Father of Haute Couture, Fashion designer (d. 1895)
October 17 – Louis Joseph Troost, French chemist (d. 1911)
October 25 – Johann Strauss, Junior, Austrian composer (d. 1899)
November 9 – A. P. Hill, American Confederate general (d. 1865)
November 29 – Jean-Martin Charcot, French physician and neurologist (d. 1893)
December 2 – Emperor Pedro II of Brazil (d. 1891)
December 30 – Samuel Newitt Wood, American politician (d. 1891)
Sher Ali Khan, ruler of Afghanistan (d. 1879)
January 4 – Ferdinand I of the Two Sicilies (b. 1751)
March 1 – John Haggin, "Indian fighter" and one of the earliest settlers of Kentucky (b. 1753)
March 4 – Hercules Mulligan, tailor and spy during the American Revolutionary War (b. 1740)
March 25 – Antoine Fabre d'Olivet, French writer (b. 1767)
May 7 – Antonio Salieri, Italian composer (b. 1750)
May 19 – Claude Henri de Rouvroy, comte de Saint-Simon, French politician (b. 1760)
May 22 – Laskarina Bouboulina, Greek independence fighter and heroine (shot) (b. 1771)
May 23 – Ras Gugsa of Yejju, Regent of the Emperor of Ethiopia
June 11 – Daniel D. Tompkins, 6th Vice President of the United States (b. 1774)
June 27 – Domenico Vantini, Italian painter
July 12 – Dorothea von Rodde-Schlözer, German scholar (b. 1770)
August 16 – Charles Cotesworth Pinckney, American politician and soldier (b. 1746)
August 20 – William Waldegrave, 1st Baron Radstock, British admiral, Governor of Newfoundland (b. 1753)
October 9 – Lucia Pytter, Norwegian philanthropist (b. 1762)
October 13 – King Maximilian I Joseph of Bavaria (b. 1756)
November 14 – Jean Paul, German writer (b. 1763)
December 1 – Emperor Alexander I of Russia (disputed; b. 1777)
December 28 – James Wilkinson, American soldier and statesman
December 29 – Jacques-Louis David, French painter (b. 1748)
Armand-Marie-Jacques de Chastenet, Marquis of Puységur, French mesmerist (b. 1751)
Huang Peilie, Chinese bibliophile (born 1763)
Maria Angela Ardinghelli, Italian scientific translator (b. 1730)
1825 (MDCCCXXV) was a common year starting on Saturday (dominical letter B) of the Gregorian calendar and a common year starting on Thursday (dominical letter D) of the Julian calendar, the 1825th year of the Common Era (CE) and Anno Domini (AD) designations, the 825th year of the 2nd millennium, the 25th year of the 19th century, and the 6th year of the 1820s decade. As of the start of 1825, the Gregorian calendar was 12 days ahead of the Julian calendar, which remained in localized use until 1923.