The Serbian hajduks (Serbian: хајдуци, hajduci) were brigands and guerrilla fighters (rebels) throughout Ottoman-held Balkans, organized into bands headed by a harambaša ("bandit leader"), who descended from the mountains and forests and robbed and attacked the Ottomans. They were often aided by foreign powers, the Republic of Venice and Habsburg Monarchy, during greater conflicts.
The hajduks are seen as part of the Serbian national identity. In stories, the hajduks were heroes; they had played the role of the Serbian elite during Ottoman rule; they had defended the Serbs against Ottoman oppression, and prepared for the national liberation and contributed to it in the Serbian Revolution. The Chetniks saw themselves as hajduks, freedom fighters.
The hajduk movement is known as hajdučija (хајдучија) or hajdukovanje (хајдуковање). Ranks included buljubaša and harambaša, adopted from the Ottomans.
Starina Novak (~1530–1601), a military commander in Wallachian service, is said to have been the oldest hajduk.
On 26 November 1716, Austrian general Nastić with 400 soldiers and c. 500 hajduks attacked Trebinje, but did not take it over. A combined Austro-Venetian-Hajduk force of 7,000 stood before the Trebinje walls, defended by 1,000 Ottomans. The Ottomans were busy near Belgrade and with hajduk attacks towards Mostar, and were thus unable to reinforce Trebinje. The conquest of Trebinje and Popovo field were given up to fight in Montenegro. The Venetians took over Hutovo and Popovo, where they immediately recruited militarly from the population.
The Serbs established a Hajduk army that supported the Austrians. The army was divided into 18 companies, in four groups. In this period, the most notable obor-kapetans were Vuk Isaković from Crna Bara, Mlatišuma from Kragujevac and Kosta Dimitrijević from Paraćin.