Thelema () is a Western esoteric and occult social or spiritual philosophy and new religious movement founded in the early 1900s by Aleister Crowley (1875–1947), an English writer, mystic, occultist, and ceremonial magician. The word thelema is the English transliteration of the Koine Greek noun θέλημα (pronounced [θelima]), "will", from the verb θέλω (thélō): "to will, wish, want or purpose."

Adherents to Thelema are called Thelemites, and phenomena within the scope of Thelema are termed Thelemic.

Crowley wrote that, in 1904, he had received a text or scripture called The Book of the Law, dictated to him by a potentially non-corporeal entity named Aiwass. This text was to serve as the foundation of the religious and philosophical system he called Thelema.

Crowley identified himself as the prophet of a new era in humanity's spiritual development, a novel age he termed the Æon of Horus. According to Crowley, the facticity of his prophethood was mainly predicated upon his reception of The Book of the Law from Aiwass, an experience that formed the crux of a broader spiritual ordeal that he and his wife, Rose Edith, underwent in Cairo, Egypt in March of 1904.

By Crowley's account, Aiwass contacted him through Rose and subsequently dictated The Book of the Law or Liber AL vel Legis, which outlined the principles of Thelema, to him.

The Thelemic pantheon—a collection of gods and goddesses who either literally exist or serve as symbolic archetypes or metaphors—includes a number of deities, primarily a trio adapted from ancient Egyptian religion, who are the three narrators or "speakers" of The Book of the Law: Nuit, Hadit, and Ra-Hoor-Khuit. In at least one instance, Crowley described these deities as a "literary convenience".

Crowley's later writings included related commentary and hermeneutics on Thelema and Thelemic topics, but also additional "inspired" writings that he collectively termed The Holy Books of Thelema. He associated Thelemic spiritual practice with concepts rooted in occultism, yoga, and Eastern and Western mysticism, especially the Qabalah.

Aspects of Thelema and Crowley's thought in general inspired the development of Wicca and, to a certain degree, the rise of Modern Paganism as a whole, as well as chaos magick and some variations of Satanism. Some scholars, such as Hugh Urban, also believe Thelema to have been an influence on the development of Scientology, but others, such as J. Gordon Melton, argue against there being a connection.

Since Crowley's death, various adherents to Thelema, including direct disciples of Crowley himself, have made significant contributions to the propagation of and discussion surrounding the religion. For example, Jack Parsons, Kenneth Grant, James Lees, and Nema Andahadna are several influential Thelemites who have arguably contributed to a broader and richer understanding of Thelemic spirituality and philosophy.