The Internal Macedonian Revolutionary Organization (IMRO; Bulgarian: Вътрешна Македонска Революционна Организация (ВМРО), Vatreshna Makedonska Revolyutsionna Organizatsiya (VMRO); Macedonian: Внатрешна Македонска Револуционерна Организација, Vnatrešna Makedonska Revolucionerna Organizacija) was a secret revolutionary society in the Ottoman territories in Europe, that operated in the late 19th and early 20th centuries.Founded in 1893 in Salonica, initially, it aimed to gain autonomy for Macedonia and Adrianople regions in the Ottoman Empire, however, later it became an agent serving Bulgarian interests in Balkan politics. IMRO group modeled itself after the Internal Revolutionary Organization of Vasil Levski and accepted its motto "Freedom or Death" (Свобода или смърть). Starting in 1896 it fought the Ottomans using guerrilla tactics, and in this they were successful, even establishing a state within a state in some regions, including their tax collectors. This effort escalated in 1903 into the Ilinden–Preobrazhenie Uprising. The fighting involved about 15,000 IMRO irregulars and 40,000 Ottoman soldiers. After the uprising failed, and the Ottomans destroyed some 100 villages, the IMRO resorted to more systematic forms of terrorism targeting civilians. During the Balkan Wars and the First World War, the organization supported the Bulgarian army and joined Bulgarian war-time authorities when they temporarily took control over parts of Thrace and Macedonia. In this period autonomism as a political tactic was abandoned and annexationist positions were supported, aiming eventual incorporation of occupied areas into Bulgaria.After the First World War the combined Macedonian-Thracian revolutionary movement separated into two detached organizations, IMRO and ITRO. After this moment the IMRO earned a reputation as an ultimate terror network, seeking to change state frontiers in the Macedonian regions of Greece and Serbia (later Yugoslavia). They contested the partitioning of Macedonia and launched raids from their Petrich stronghold into Greek and Yugoslav territory. Their base of operation in Bulgaria was jeopardized by the Treaty of Niš, and the IMRO reacted by assassinating Bulgarian prime minister Aleksandar Stamboliyski in 1923, with the cooperation of other Bulgarian elements opposed to him. In 1925 the Greek army launched a cross-border operation to reduce the IMRO base area, but it was ultimately stopped by the League of Nations, and IMRO attacks resumed. In the interwar period the IMRO also cooperated with the Croatian Ustaše, and their ultimate victim was Alexander I of Yugoslavia, assassinated in France in 1934. After the Bulgarian coup d'état of 1934, their Petrich stronghold was subjected to military crackdown by the Bulgarian army, and the IMRO was reduced to a marginal phenomenon.

The organization changed its name on several occasions. After the fall of communism in the region, numerous parties claimed the IMRO name and lineage to legitimize themselves. Among them, in Bulgaria a right-wing party carrying the prefix "VMRO" was established in the 1990s, while in the Republic of Macedonia a right-wing party was established under the name "VMRO-DPMNE".