Marxist philosophy or Marxist theory are terms that cover work in philosophy that is strongly influenced by Karl Marx's materialist approach to theory or that is written by Marxists. It may be broadly divided into Western Marxism, which drew out of various sources, and the official philosophy in the Soviet Union, which enforced a rigid reading of Marx called "diamat" (for "dialectical materialism"), in particular during the 1930s.

The phrase "Marxist philosophy" itself does not indicate a strictly defined sub-field of philosophy, because the diverse influence of Marxist theory has extended into fields as varied as aesthetics, ethics, ontology, epistemology, and philosophy of science, as well as its obvious influence on political philosophy and the philosophy of history. The key characteristics of Marxism in philosophy are its materialism and its commitment to political practice as the end goal of all thought.

Louis Althusser, for example, defined philosophy as "class struggle in theory", thus radically separating himself from those who claimed philosophers could adopt a "God's eye view" as a purely neutral judge. Just as the young Marx had left university and German Idealism to encounter the proletariat, which permitted him to modify his perspective on practice and theory, "intellectuals" couldn't content themselves with instructing the masses from their chairs (as the "organic intellectual" conception denounced by Antonio Gramsci) but personally had to take part in the social struggles of their times.