October 27 is the 300th day of the year (301st in leap years) in the Gregorian calendar. There are 65 days remaining until the end of the year.
* 312 – Constantine the Great is said to have received his famous Vision of the Cross.
* 710 – Saracen invasion of Sardinia.
* 939 – Edmund I succeeds Athelstan as King of England.
* 1275 – Traditional founding of the city of Amsterdam.
* 1524 – Italian Wars: The French troops lay siege to Pavia.
* 1553 – Condemned as a heretic, Michael Servetus is burned at the stake just outside Geneva.
* 1644 – Second Battle of Newbury in the English Civil War.
* 1682 – Philadelphia, Pennsylvania is founded.
* 1795 – The United States and Spain sign the Treaty of Madrid, which establishes the boundaries between Spanish colonies and the U.S.
* 1806 – The French Army enters Berlin.
* 1810 – United States annexes the former Spanish colony of West Florida.
* 1838 – Missouri governor Lilburn Boggs issues the Extermination Order, which orders all Mormons to leave the state or be exterminated.
* 1870 – Marshal François Achille Bazaine surrenders to Prussian forces at Metz along with 140,000 French soldiers in one of the biggest French defeats of the Franco-Prussian War.
* 1904 – The first underground New York City Subway line opens; the system becomes the biggest in United States, and one of the biggest in world.
* 1914 – World War I: The British super-dreadnought battleship HMS Audacious (23,400 tons), is sunk off Tory Island, north-west of Ireland, by a minefield laid by the armed German merchant-cruiser Berlin.
* 1916 – Battle of Segale: Negus Mikael, marching on the Ethiopian capital in support of his son Emperor Iyasus V, is defeated by Fitawrari abte Giyorgis, securing the throne for Empress Zauditu.
* 1922 – A referendum in Rhodesia rejects the country's annexation to the South African Union.
* 1924 – The Uzbek SSR is founded in the Soviet Union.
* 1936 – Mrs Wallis Simpson files for divorce which would eventually allow her to marry King Edward VIII of the United Kingdom, thus forcing his abdication from the throne.
* 1948 – Léopold Sédar Senghor founds the Senegalese Democratic Bloc.
* 1953 – British nuclear test Totem 2 is carried out at Emu Field, South Australia.
* 1954 – Benjamin O. Davis Jr. becomes the first African-American general in the United States Air Force.
* 1958 – Iskander Mirza, the first President of Pakistan, is deposed in a bloodless coup d'état by General Ayub Khan, who had been appointed the enforcer of martial law by Mirza 20 days earlier.
* 1961 – NASA launches the first Saturn I rocket in Mission Saturn-Apollo 1.
* 1961 – Mauritania and Mongolia join the United Nations.
* 1962 – Major Rudolf Anderson of the United States Air Force becomes the only direct human casualty of the Cuban Missile Crisis when his U-2 reconnaissance airplane is shot down in Cuba by a Soviet-supplied SA-2 Guideline surface-to-air missile.
* 1962 – A plane carrying Enrico Mattei, post-war Italian administrator, crashes in mysterious circumstances.
* 1964 – Ronald Reagan delivers a speech on behalf of Republican candidate for president, Barry Goldwater. The speech launched his political career and came to be known as "A Time for Choosing".
* 1967 Catholic priest Philip Berrigan and others of the Baltimore Four protest the Vietnam War by pouring blood on Selective Service records.
* 1971 – The Democratic Republic of the Congo is renamed Zaire.
* 1973 – The Cañon City meteorite, a 1.4 kg chondrite type meteorite, strikes in Fremont County, Colorado.
* 1981 – The Soviet submarine U 137 runs aground on the east coast of Sweden.
* 1986 – The British government suddenly deregulates financial markets, leading to a total restructuring of the way in which they operate in the country, in an event now referred to as the Big Bang.
* 1988 – Ronald Reagan decides to tear down the new U.S. Embassy in Moscow because of Soviet listening devices in the building structure.
* 1991 – Turkmenistan achieves independence from the Soviet Union.
* 1992 – United States Navy radioman Allen R. Schindler, Jr. is brutally murdered by shipmate Terry M. Helvey for being gay, precipitating first military, then national, debate about gays in the military that resulted in the United States "Don't ask, don't tell" military policy.
* 1994 – The U.S. prison population tops 1 million for the first time in American history.
* 1994 – Gliese 229B is the first Substellar Mass Object to be unquestionably identified.
* 1995 – Latvia applies for membership in the European Union.
* 1995 – Former Prime Minister of Italy Bettino Craxi is convicted in absentia of corruption.
* 1997 – October 27, 1997 mini-crash: Stock markets around the world crash because of fears of a global economic meltdown. The Dow Jones Industrial Average plummets 554.26 points to 7,161.15. For the first time, the New York Stock Exchange activates its "circuit breakers" twice during the day eventually making the controversial move of closing the Exchange early.
* 1999 – Gunmen open fire in the Armenian Parliament, killing Prime Minister Vazgen Sargsyan, Parliament Chairman Karen Demirchyan, and 6 other members.
* 2004 – The Boston Red Sox win the World Series for the first time in 86 years.
* 2005 – Riots begin in Paris after the deaths of two Muslim teenagers.
* 2005 – The SSETI Express micro-satellite is successfully launched from the Plesetsk Cosmodrome.