American author, attorney, philosopher, and cofounder of the Theosophical Society, a religious sect incorporating aspects of Buddhism, Brahmanism, and Christian esotericism.

Olcott was agricultural editor of the New York Tribune (1858–60), and with the rank of colonel he was special commissioner in the U.S. War and Navy departments (1863–66). He was admitted to the bar in 1866. With Helena Petrovna Blavatsky, William Q. Judge, and others he founded the Theosophical Society in 1875 and became its president. In 1878 he and Blavatsky visited India. The two settled there in 1879 and in 1882 established the permanent headquarters of the Theosophical Society of Adyar, Madras. He assisted Annie Besant (Besant, Annie) in establishing the Central Hindu College at Vārānasi (Benares). With her, he expounded their Theosophist ideas in appearances in India and Ceylon. Urging educational advancement upon Ceylon Buddhists, he saw three colleges and 250 schools established as a result of his efforts. His acceptance by and influence on the Buddhists was far-reaching. Identified with Eastern philosophical thought, he also helped revive Hindu philosophy; a pandit conferred on him the sacred thread of the Brahman caste.

Olcott edited the Theosophist (1888–1907). His Buddhist Catechism (1881) was translated into many languages.