Undercover is a 1943 British war film produced by Ealing Studios, originally titled Chetnik. It was filmed in Wales and released on 27 July 1943. Its subject is a guerrilla movement in German-occupied Yugoslavia, loosely based on Draza Mihailovich's Chetnik resistance movement.

The film was produced by Michael Balcon and directed by Sergei Nolbandov. It stars John Clements, Mary Morris, and Stephen Murray, with Michael Wilding and 15-year-old Stanley Baker.

The film was released in the United States in 1944 by Columbia Pictures under the title Underground Guerrillas. It is similar to the 20th Century Fox wartime film Chetniks! The Fighting Guerrillas (1943), made in the U.S.