反帝国主义严格而言是指反对任何帝国主义和殖民主义形式的运动，而广义上也反对侵略战争，尤其是对拥有不同文化，且非邻近国家之间的侵犯。起源于19世纪后期与20世纪初期对于欧洲势力扩张的反抗。是20世纪亚洲与非洲国家逐渐脱离殖民统治时所盛行的政治思想之一。而较为晚近的当代，全球化也被部分的反帝国主义者看作是一种帝国主义，进而成为反帝国主义的主要反抗对象，导致反全球化运动诞生。除此之外，当代的一些军事冲突，例如美国对伊拉克的战争，也是反帝国主义者的抗争目标。

Anti-imperialism in political science and international relations is a term used in a variety of contexts, usually by nationalist movements, who want to secede from a larger polity (usually in the form of an empire, but also in a multi-ethnic sovereign state) or as a specific theory opposed to capitalism in Marxist–Leninist discourse, derived from Vladimir Lenin's work Imperialism, the Highest Stage of Capitalism. A less common usage is by isolationists who oppose an interventionist foreign policy.