September 17 is the 260th day of the year (261st in leap years) in the Gregorian calendar; 105 days remain until the end of the year.
1111 – Highest Galician nobility led by Pedro Fróilaz de Traba and the bishop Diego Gelmírez crown Alfonso VII as 'King of Galicia' 1176 – The Battle of Myriokephalon is the last attempt by the Byzantine Empire to recover central Anatolia from the Seljuk Turks.
1382 – Louis the Great's daughter, Mary, is crowned 'king' of Hungary.
1462 – Thirteen Years' War: A Polish army under Piotr Dunin decisively defeats the Teutonic Order at the Battle of Świecino.
1543 – The first Finnish-language book, the Abckiria by Mikael Agricola, is published in Stockholm.
1577 – The Treaty of Bergerac is signed between King Henry III of France and the Huguenots.
1620 – Polish–Ottoman War: The Ottoman Empire defeats the Polish–Lithuanian Commonwealth during the Battle of Cecora.
1630 – The city of Boston, Massachusetts, is founded in North America.
1631 – Sweden wins a major victory at the Battle of Breitenfeld against the Holy Roman Empire during the Thirty Years' War.
1658 – The Battle of Vilanova is fought between Portugal and Spain during the Portuguese Restoration War.
1683 – Antonie van Leeuwenhoek writes a letter to the Royal Society describing 'animalcules', later known as protozoa.
1775 – American Revolutionary War: The invasion of Quebec by the Continental Army begins with the Siege of Fort St. Jean.
1776 – The Presidio of San Francisco is founded in New Spain.
1778 – The Treaty of Fort Pitt is signed. It is the first formal treaty between the United States and a Native American tribe.
1787 – The United States Constitution is signed at Independence Hall in Philadelphia, bringing the Constitutional Convention to an end.
1793 – War of the Pyrenees: France defeats a Spanish force at the Battle of Peyrestortes.
1794 – Flanders Campaign: France completes its conquest of the Austrian Netherlands at the Battle of Sprimont.
1809 – Peace between Sweden and Russia in the Finnish War; the territory that will become Finland is ceded to Russia by the Treaty of Fredrikshamn.
1849 – American abolitionist Harriet Tubman escapes from slavery.
1859 – Joshua A. Norton declares himself 'Norton I, Emperor of the United States'.
1861 – Argentine Civil Wars: The State of Buenos Aires defeats the Argentine Confederation at the Battle of Pavón.
1862 – American Civil War: George B. McClellan's Army of the Potomac halts the first invasion of the North by Robert E. Lee and his Army of Northern Virginia in the single-day Battle of Antietam, the bloodiest day in American military history.
1862 – American Civil War: The Allegheny Arsenal explosion in Lawrenceville, Pennsylvania results in the single largest civilian disaster during the war.
1894 – Battle of the Yalu River, the largest naval engagement of the First Sino-Japanese War 1900 – Philippine– American War: Filipinos under Juan Cailles defeat Americans under Colonel Benjamin F. Cheatham Jr. at Mabitac.