The Chetniks (Serbian: Четници, Četnici) were a Royalist paramilitary formations operating in the Balkans before and during World Wars. During World War II, the Chetniks were known officially as "Yugoslav Army in the Fatherland" (Југословенска војска у отаџбини, Jugoslovenska vojska u otadžbini, or ЈВУО, JVUO), and consisted mostly of Serbs loyal to the Kingdom of Yugoslavia's government in exile. The name "Chetnik" was also used by some guerilla squads active in the wars in the Balkans prior to World War I. The name chetnik is derived from the Serbian word četa (чета) which means "military company", itself derived from Turkish çete, gang or band (e.g., of brigands).

The role of Chetniks during the World War II remains a controversial issue in former Yugoslav countries. While in Socialist Yugoslavia they were condemned as quislings and war criminals—the characterization also supported by Croatian and Bosnia and Herzegovinan historians and public perception—in today's Serbia they are largely rehabilitated.

In modern times, especially during and after Yugoslav Wars, "chetnik" come to be used as an ethnic slur against Serbs. However, some Serb nationalist and paramilitary organizations self-identified with the term, .