Chetniks, or the Chetnik movement (Serbian: Četnici, Четници, pronounced [tʃɛ̂tniːtsi]), were Serbian nationalist and monarchist paramilitary organizations from the first half of the 20th century, formed as a resistance against the Ottoman Empire in 1904, and participating in the two Balkan Wars,World War I, and World War II. Between the wars, in the Kingdom of Yugoslavia, they functioned in the form of two civilian organizations. The name is today most closely associated with the Chetnik Detachments of the Yugoslav Army, the World War II movement of Draža Mihailović, which was later renamed the Yugoslav Army in the Homeland (Jugoslovenska vojska u otadžbini, Југословенска војска у отаџбини; JVUO, ЈВУО), though the original name remained more common. The Mihailović Chetniks were not a homogeneous movement.

During World War II, the Chetniks were an anti-Axis movement in their long-range goals and engaged in marginal resistance activities for limited periods, but also carried out almost throughout the war a tactical or selective collaboration with the occupation. The Chetnik movement collaborated with the Axis occupation to one degree or another by establishing modus vivendi or operating as "legalised" auxiliary forces under Axis control. Over a period of time, and in different parts of the country, the Chetnik movement was progressively drawn into collaboration agreements: first with the Nedić forces in Serbia, then with the Italians in occupied Dalmatia and Montenegro, with some of the Ustaše forces in northern Bosnia, and after the Italian capitulation also with the Germans directly. While Chetnik collaboration reached "extensive and systematic" proportions, the Chetniks themselves referred to their policy of collaboration as "using the enemy".