British historian of religion who investigated the evolution of ethical systems and forms of mysticism, particularly in Eastern religions.

The son of Swiss parents who had immigrated to England, Zaehner studied Oriental languages at the University of Oxford, specializing in Persian, Armenian, and Avestan. He became a Roman Catholic in 1946. During and immediately after World War II, he served in the British Army as a press attaché in Tehrān. After resuming his academic career, he succeeded Sir Sarvepalli Radakrishnan as Spalding Professor of Eastern Religions and Ethics at the University of Oxford (1952), enduring criticism at the time because the chair was considered reserved for Asians.

With his magisterial Zurvan; a Zoroastrian Dilemma (1955), he began publishing the results of a lifelong study in the history of religions, including Mysticism Sacred and Profane (1957), Hindu and Muslim Mysticism (1960), Hinduism (1962), and a translation of Hindu scriptures (1966). Other works on Zoroastrianism include The Teachings of the Magi (1956) and The Dawn and Twilight of Zoroastrianism (1961).

Among his later works were Evolution in Religion (1971); Dialectical Christianity and Christian Materialism (1971); and Drugs, Mysticism, and Make Believe (1972). His last book, Our Savage God (1974), a philosophical and somewhat provocative view of the ills of modern society, aroused controversy among its critics.