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1825 (MDCCCXXV) was a common year starting on Saturday of the Gregorian calendar and a common year starting on Thursday 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.
- 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.
- June 2 – The United States Senate ratifies the treaties with the Great Osage and the Little Osage tribes. 
- June 3 – The U.S. Senate ratifies the treaty with the Kansas tribe. 
- June 9 – The U.S. Senate ratifies the treaty with the Poncas tribe. 
- June 15 – A rebellion is started by 200 slaves in the Guamacaro region of Cuba, and is suppressed after 12 hours; in the ensuing months, most who weren't killed in the battle would be hunted down and killed. 
- 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 6 – The U.S. Senate ratifies treaties with the Cheyenne tribe. 
- July 16 – The U.S. Senate ratifies treaties with the Hunkpapa tribe. 
- July 18 – Uruguay secedes from Brazil.
- July 18 – The U.S. Senate ratifies treaties with the Ricara tribes. 
- July 30 – Malden Island (an uninhabited island in the central Pacific Ocean) is discovered by George Byron, 7th Baron Byron.
- July 30 – The U.S. Senate ratifies treaties with the Mandan, Belantae, Eloa and Minnetaree tribes. 
- August 4 – The U.S. Senate ratifies treaties with the Ricara tribes. 
- August 6 – Bolivia gains its independence from Spain as a republic, with the instigation of Simón Bolívar.
- August 11 – The U.S. Senate ratifies treaties with the Crow tribe. 
- 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 22 – The National Mexican Rite is created in Mexico City.
- 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 26 – The U.S. Senate ratifies treaties with the Missouri and Ottoe tribes. 
- September 27 – The world's first modern railway, the Stockton and Darlington Railway, opens in England.
- September 30 – The U.S. Senate ratifies treaties with the Pawnee tribe.