Armatoloi (pronounced ar-ma-to-LEE; Greek plural Αρματολοί; singular Armatolos, Αρματολός; also called Armatoles in English) were Christian Greek irregular soldiers, or militia, commissioned by the Ottomans to enforce the Sultan's authority within an administrative district called an Armatoliki (Greek singular Αρματολίκι; plural Armatolikia, Αρματολίκια). Armatolikia were created in areas of Greece that had high levels of brigandage (i.e. klephts), or in regions that were difficult for Ottoman authorities to govern due to the inaccessible terrain, such as the Agrafa mountains of Thessaly, where the first armatoliki was established in the 15th century. Over time, the roles of the armatoloi and klephtes became blurred, with both reversing their roles and allegiances as the situation demanded, all the while maintaining the delicate status-quo with the Ottoman authorities.
During the Greek War of Independence, the armatoloi, along with the klephts, formed the nucleus of the Greek fighting forces, and played a prominent part throughout its duration. Yannis Makriyannis referred to the armatoloi and klephts as the "yeast of liberty" (μαγιά της λευτεριάς).
The word armatolos first appeared in the 15th century during Venetian times. It is derived from a medieval loan from Latin arma ('weapon'), probably via Greek αρματολόγος ('someone who deals with arms', 'an armed person') → αρματολόος → αρματολός. According to an older hypothesis, the development of the word may also have been influenced by a conflation with the similar-sounding αμαρτωλός ("sinner"; cf. hamartia), which may have been associated with the topic of armed bands through phrases such as "αμαρτωλοί/αρματολοί και κλέφτες" (meaning "sinners and thieves", but also "armatoloi and klephts"). Owing to the parallelism with "αμαρτωλός", the word was also sometimes spelled as "αρματωλός", with the letter omega.