Rajneesh (born Chandra Mohan Jain; 11 December 1931 – 19 January 1990), also known as Acharya Rajneesh, and commonly known as Osho (Hindi: [ˈoːʃoː]), was an Indian godman, philosopher, mystic, and founder of the Rajneesh movement. He was a controversial new religious movement leader during his life. He rejected institutional religions, insisting that spiritual experience could not be organized into any one system of religious dogma. As a guru, he advocated meditation and taught a unique form called dynamic meditation. Rejecting traditional ascetic practices, he encouraged his followers to embrace life fully while remaining unattached to worldly desires.

Rajneesh experienced a spiritual awakening in 1953 at the age of 21. Following several years in academia, in 1966 Rajneesh resigned his post at the University of Jabalpur as a lecturer in philosophy, and began traveling throughout India, becoming known for his criticism of the orthodoxy of mainstream religions, as well as of mainstream political ideologies and of Mahatma Gandhi. In 1970, Rajneesh spent time in Mumbai initiating followers referred to as "neo-sannyasins". During this period, he expanded his spiritual teachings and commented extensively in discourses on the writings of religious traditions, mystics, bhakti poets, and philosophers from around the world. In 1974, Rajneesh relocated to Pune, where an ashram was established and a variety of therapies, incorporating methods first developed by the Human Potential Movement, were offered to a growing Western following. By the late 1970s, the tension^{[clarification needed]} between the ruling Janata Party government of Morarji Desai and the movement led to a curbing of the ashram's development and a back tax claim estimated at $5 million.

In 1981, the Rajneesh movement shifted its focus to the United States, where Rajneesh established a commune called Rajneeshpuram in Wasco County, Oregon. The movement ran into conflict with county residents and the state government, and a succession of legal battles concerning the ashram's construction and continued development curtailed its success. In 1985, Rajneesh publicly asked local authorities to investigate his personal secretary Ma Anand Sheela and her close supporters for a number of crimes, including a 1984 mass food-poisoning attack intended to influence county elections, an aborted assassination plot on U.S. attorney Charles H. Turner, the attempted murder of Rajneesh's personal physician, and the bugging of his own living quarters; authorities later convicted several members of the ashram, including Sheela. That year, Rajneesh was deported from the United States on separate immigration-related charges in accordance with an Alford plea. After his deportation, 21 countries denied him entry.

Rajneesh returned to Mumbai in 1986 and, after a period of public discourse, moved back to Pune in January 1987 to revive his ashram, where he died in 1990. Rajneesh's ashram, now known as OSHO International Meditation Resort, and all associated intellectual property, is managed by the registered Osho International Foundation (formerly Rajneesh International Foundation). Rajneesh's teachings have had an impact on Western New Age thought, and their popularity increased after his death.

Rajneesh (a childhood nickname from the Sanskrit रजनी, rajanee, "night", and ईश, isha, "lord") was born Chandra Mohan Jain, the eldest of 11 children of a cloth merchant, at his maternal grandparents' house in Kuchwada; a small village in the Raisen District of Madhya Pradesh state in India. Babulal and Saraswati Jain, his parents, were Jain's and followers of Taran Svami, and they arranged for him to live with his maternal grandparents until the age of eight. According to Rajneesh, this profoundly influenced his development, as his grandmother gave him complete freedom from imposed education or restrictions.

When he was seven years old, his grandfather died, and he went to Gadarwara to live with his parents. Rajneesh was profoundly affected by his grandfather's death, and again by the death of his childhood girlfriend Shashi from typhoid when he was 15, leading to a preoccupation with death that lasted throughout much of his childhood and youth. In his school years, he was a gifted and rebellious student, and gained a reputation as a formidable debater. Rajneesh became critical of traditional religion, took an interest in many methods to expand consciousness, including breath control, yogic exercises, meditation, fasting, the occult, and hypnosis. According to Vasant Joshi, Rajneesh read widely from an early age; although he played sports as a young boy, reading was his primary interest. After showing an interest in the writings of Marx and Engels, he was branded a communist and was threatened with expulsion from school. According to Joshi, with the help of friends, he built a small library containing mostly communist literature. Rajneesh, according to his uncle Amritlal, also formed a group of young people that regularly discussed communist ideology and their opposition to religion.

Rajneesh was later to say, "I have been interested in communism from my very childhood...communist literature — perhaps there is no book that is missing from my library. I have signed and dated each book before 1950. Small details are so vivid before me, because that was my first entry into the intellectual world. First I was deeply interested in communism, but finding that it is a corpse I became interested in anarchism — that was also a Russian phenomenon — Prince Kropotkin, Bakunin, Leo Tolstoy. All three were anarchists: no state, no government in the world."

He became briefly associated with socialism and two Indian nationalist organisations: the Indian National Army and the Rashtriya Swayamsevak Sangh. However, his membership in the organisations was short-lived as he could not submit to any external discipline, ideology, or system.