Fifth column refers to any clandestine group of people which works covertly inside a nation to undermine its strength (psychological warfare) while the nation is simultaneously suffering an overt attack by a foreign power or another faction in a civil war.

The term was coined by General Emilio Mola in a radio address during the Spanish Civil War (1936–1939). As a Nationalist general, he sent four army columns to capture Madrid, which was being defended at that time by the Republican forces. The general referred to his hidden supporters inside the capital as his fifth column. The term caught on after the address was widely reported by international media.

The only play written by Ernest Hemingway, entitled The Fifth Column, depicts the roles of the two protagonists — a writer and a journalist — during wartime. The title hints at the similarity of the protagonists with the supporters of Emilio Mola, in that both were attempting to influence events while performing "behind enemy lines".

The term is also used in reference to immigrants, who are assumed to have loyalties other than to the countries in which they reside. For example Jews in early 20th Century Britain were suspected of being a disloyal fifth column, thought to be loyal to 'European' Anarchism and Bolshevism (the feared ideologies of the day). Today some on the Right in Britain see Muslims as being a fifth column of a global Islamist movement. Those on the Left generally see use of the term in this context as scaremongering. For discussion of this usage see Jonathan Freedland's article in the Guardian [1]

Leon Trotsky originally used "Fifth Column" as his name for what would become the Fifth Army, an elite military unit founded during the Russian Civil War. This was not a covert force, and has little in common with the usual use of the term.

Fifth Column is also the name of an all-girl punk band from Toronto. There was also a punk band called Fifth Column from San Francisco in the 1980s