The perennial philosophy (Latin: philosophia perennis), also referred to as perennialism and perennial wisdom, is a school of thought in philosophy and spirituality that posits that the recurrence of common themes across world religions illuminates universal truths about the nature of reality, humanity, ethics, and consciousness. Some perennialists emphasize common themes in religious experiences and mystical traditions across time and cultures; others argue that religious traditions share a single metaphysical truth or origin from which all esoteric and exoteric knowledge and doctrine have developed.

Perennialism has its roots in the Renaissance-era interest in neo-Platonism and its idea of the One from which all existence emerges. Marsilio Ficino (1433–1499) sought to integrate Hermeticism with Greek and Christian thought, discerning a prisca theologia found in all ages. Giovanni Pico della Mirandola (1463–1494) suggested that truth could be found in many—rather than just Biblical and Aristotelian traditions. He proposed a harmony between the thought of Plato and Aristotle and saw aspects of the prisca theologia in Averroes (Ibn Rushd), the Quran, Kabbalah, and other sources. Agostino Steuco (1497–1548) coined the term philosophia perennis.

Developments in the 19th and 20th centuries integrated Eastern religions and universalism—the idea that all religions, underneath apparent differences, point to the same Truth. In the early 19th century, the Transcendentalists propagated the idea of a metaphysical Truth and universalism—this inspired the Unitarians, who proselytized among Indian elites. Toward the end of the 19th century, the Theosophical Society further popularized universalism in the Western world and Western colonies. In the 20th century, this form of universalist perennialism was further popularized by Aldous Huxley and his book The Perennial Philosophy, which was inspired by Neo-Vedanta. Huxley and some other perennialists grounded their point of view in the commonalities of mystical experience and generally accepted religious syncretism.

Also, in the 20th century, the anti-modern Traditionalist School emerged in contrast to the universalist approach to perennialism. Inspired by Advaita Vedanta, Sufism and 20th-century works critical of modernity such as René Guénon's The Crisis of the Modern World, Traditionalism emphasises a metaphysical unitary source of the major religions in their "orthodox" forms and rejects syncretism, scientism, and secularism as deviations from the truth contained in their concept of Tradition.

There is no universally agreed upon definition of the term "perennial philosophy", and various thinkers have employed the term in different ways. For all perennialists, the term denotes a common wisdom at the heart of world religions, but exponents across time and place have differed on whether, or how, it can be defined. Some perennialists emphasise a sense of participation in an ineffable truth discovered in mystical experience, though ultimately beyond the scope of complete human understanding. Others seek a more well-developed metaphysics.

Drawing upon the same Renaissance foundations, in the 20th century the mystical universalist interpretation popularised by Aldous Huxley, and the metaphysical approach of the Traditionalist School became particularly influential.

The idea of a perennial philosophy originated with a number of Renaissance theologians who took inspiration from neo-Platonism and from the theory of Forms. Marsilio Ficino (1433–1499) argued that there is an underlying unity to the world, the soul or love, which has a counterpart in the realm of ideas. According to Giovanni Pico della Mirandola (1463–1494), a student of Ficino, truth could be found in many, rather than just two, traditions. According to Agostino Steuco (1497–1548) there is "one principle of all things, of which there has always been one and the same knowledge among all peoples."

Aldous Huxley, author of the popular book The Perennial Philosophy, propagated a universalist interpretation of the world religions, inspired by Vivekananda's neo-Vedanta and his own use of psychedelic drugs. According to Huxley: