In late modern philosophy, neo-Kantianism (German: Neukantianismus) was a revival of the 18th-century philosophy of Immanuel Kant. The neo-Kantians sought to develop and clarify Kant's theories, particularly his concept of the thing-in-itself and his moral philosophy.

The "back to Kant" movement began in the 1860s, as a reaction to the German materialist controversy in the 1850s.

In addition to the work of Hermann von Helmholtz and Eduard Zeller, early fruits of the movement were Kuno Fischer's works on Kant and Friedrich Albert Lange's History of Materialism (Geschichte des Materialismus, 1873–75), the latter of which argued that transcendental idealism superseded the historic struggle between material idealism and mechanistic materialism. Fischer was earlier involved in a dispute with the Aristotelian idealist Friedrich Adolf Trendelenburg concerning the interpretation of the results of the Transcendental Aesthetic, a dispute that prompted Hermann Cohen's 1871 seminal work Kants Theorie der Erfahrung (Kant's Theory of Experience), a book often regarded as the foundation of 20th-century neo-Kantianism. It is in reference to the Fischer–Trendelenburg debate and Cohen's work that Hans Vaihinger started his massive commentary on the Critique of Pure Reason.

The several schools of thought, in spite of seeing themselves as united by a common movement, often saw massive fundamental disagreements.

Hermann Cohen became the leader of the Marburg School (centered in the town of the same name), the other prominent representatives of which were Paul Natorp and Ernst Cassirer. Another important group, the Southwest (German) School (also known as the Heidelberg School or Baden School, centered in Heidelberg, Baden in Southwest Germany) included Wilhelm Windelband, Heinrich Rickert, Ernst Troeltsch, and Emil Lask. The Marburg School emphasized epistemology and philosophical logic, whereas the Southwest school emphasized issues of culture and value theory (notably the fact–value distinction).

A third group, mainly represented by Leonard Nelson, established the Göttingen-based neo-Friesian School (named after post-Kantian philosopher Jakob Friedrich Fries) which emphasized philosophy of science.

The neo-Kantian schools tended to emphasize scientific readings of Kant, often downplaying the role of intuition in favour of concepts. However, the ethical aspects of neo-Kantian thought often drew them within the orbit of socialism, and they had an important influence on Austromarxism and the revisionism of Eduard Bernstein. Lange and Cohen in particular were keen on this connection between Kantian thought and socialism. Another important aspect of the neo-Kantian movement was its attempt to promote a revised notion of Judaism, particularly in Cohen's seminal work, one of the few works of the movement available in English translation.

The neo-Kantian school was of importance in devising a division of philosophy that has had durable influence well beyond Germany. It made early use of terms such as epistemology and upheld its prominence over ontology. Natorp had a decisive influence on the history of phenomenology and is often credited with leading Edmund Husserl to adopt the vocabulary of transcendental idealism. Emil Lask was influenced by Edmund Husserl's work, and himself exerted a remarkable influence on the young Martin Heidegger. The debate between Cassirer and Heidegger over the interpretation of Kant led the latter to formulate reasons for viewing Kant as a forerunner of phenomenology; this view was disputed in important respects by Eugen Fink. An abiding achievement of the neo-Kantians was the founding of the journal Kant-Studien, which still survives today.