The Ustaše (pronounced [ûstaʃe]), also known by anglicised versions Ustasha or Ustashe, was a Croatian fascist and ultranationalist organization active, as one organization, between 1929 and 1945, formally known as the Ustaša – Croatian Revolutionary Movement (Croatian: Ustaša – Hrvatski revolucionarni pokret). From its inception and before the Second World War, the organization engaged in a series of terrorist activities against the Kingdom of Yugoslavia, including collaborating with IMRO to assassinate King Alexander I of Yugoslavia in 1934. During World War II in Yugoslavia, the Ustaše went on to perpetrate the Holocaust and genocide against its Jewish, Serb and Roma populations, killing hundreds of thousands of Serbs, Jews, Roma, as well as Muslim and Croat political dissidents.

The ideology of the movement was a blend of fascism, Roman Catholicism and Croatian ultranationalism. The Ustaše supported the creation of a Greater Croatia that would span the Drina River and extend to the border of Belgrade. The movement advocated a racially "pure" Croatia and promoted genocide against Serbs—due to the Ustaše's anti-Serb sentiment—and Holocaust against Jews and Roma via Nazi racial theory, and persecution of anti-fascist or dissident Croats and Bosniaks. The Ustaše viewed the Bosniaks as "Muslim Croats", and as a result, Bosniaks were not persecuted on the basis of race. The Ustaše espoused Roman Catholicism and Islam as the religions of the Croats and condemned Orthodox Christianity, which was the main religion of the Serbs. Roman Catholicism was identified with Croatian nationalism, while Islam, which had a large following in Bosnia and Herzegovina, was praised by the Ustaše as the religion that "keeps true the blood of Croats."

It was founded as a nationalist organization that sought to create an independent Croatian state. It functioned as a terrorist organization before World War II. After the invasion of Yugoslavia in April 1941, the Ustaše came to power when they were appointed to rule a part of Axis-occupied Yugoslavia as the Independent State of Croatia (NDH), a quasi-protectorate puppet state established by Fascist Italy and Nazi Germany. The Ustaše Militia (Croatian: Ustaška vojnica) became its military wing in the new state.

The Ustaše regime was militarily weak and failed to ever attain significant support among Croats. Therefore, terror was their means of controlling the "ethnically disparate" population. The Ustaše regime was initially backed by some parts of the Croat population that in the interwar period had felt oppressed by the Serb-led Yugoslavia, but their brutal policies quickly alienated many ordinary Croats and resulted in a loss of the support they had gained by creating a Croatian national state.

With the German surrender, end of World War II in Europe, and the establishment of socialist Yugoslavia in 1945, the Ustaše movement and their state totally collapsed. Many members of the Ustaše militia and Croatian Home Guard who subsequently fled the country were taken as prisoners of war and subjected to forced marches and executions during the Bleiburg repatriations. Various underground and exile successor organisations created by former Ustaše members, such as the Crusaders and the Croatian Liberation Movement, tried to continue the movement to little success.