Pyotr Demianovich Ouspenskii (known in English as Peter D. Ouspensky, 5 March 1878 – 2 October 1947), was a Russian mathematician and esotericist known for his expositions of the early work of the Greek-Armenian teacher of esoteric doctrine George Gurdjieff, whom he met in Moscow in 1915. He was associated with the ideas and practices originating with Gurdjieff from then on. He shared the (Gurdjieff) "system" for 25 years in England and the United States, having separated from Gurdjieff in 1924 personally, for reasons he explains in the last chapter of his book In Search of the Miraculous.

All in all, Ouspensky studied the Gurdjieff system directly under Gurdjieff's own supervision for a period of ten years, from 1915 to 1924. His book In Search of the Miraculous is a recounting of what he learned from Gurdjieff during those years. While lecturing in London in 1924, he announced that he would continue independently the way he had begun in 1921. Some, including his close pupil Rodney Collin, say that he finally gave up the system in 1947, just before his death, but his own recorded words on the subject ("A Record of Meetings", published posthumously) do not clearly endorse this judgement, nor does Ouspensky's emphasis on "you must make a new beginning" after confessing "I've left the system".