Satanism refers to a group of religious, ideological, or philosophical beliefs based on Satan—particularly his worship or veneration. Because of the ties to the historical Abrahamic religious figure, Satanism—as well as other religious, ideological, or philosophical beliefs that align with Satanism—is considered a countercultural Abrahamic religion.

The phenomenon of Satanism shares "historical connections and family resemblances" with the Left Hand Path milieu of other occult figures such as Asmodeus, Beelzebub, Hecate, Lilith, Lucifer, Mephistopheles, Pan, Prometheus, Samael, and Set. Self-identified Satanism is a relatively modern phenomenon, largely attributed to the 1966 founding of the Church of Satan by Anton LaVey in the United States—an atheistic group that does not believe in a supernatural Satan.

Accusations of groups engaged in "devil worship" have echoed throughout much of Christian history. During the Middle Ages, the Inquisition led by the Catholic Church alleged that various heretical Christian sects and groups, such as the Knights Templar and the Cathars, performed secret Satanic rituals. In the subsequent Early Modern period, belief in a widespread Satanic conspiracy of witches resulted in the trials and executions of tens of thousands of alleged witches across Europe and the North American colonies, peaking between 1560 and 1630. The terms Satanist and Satanism emerged during the Reformation and Counter-Reformation (1517–1700), as both Catholics and Protestants accused each other of intentionally being in league with Satan.

Since the 19th century various small religious groups have emerged that identify as Satanist or use Satanic iconography. While the groups that appeared after the 1960s differed greatly, they can be broadly divided into nontheistic Satanism and theistic Satanism. Those venerating Satan as a supernatural deity are unlikely to ascribe omnipotence, instead relating to Satan as a patriarch. Nontheistic Satanists regard Satan as a symbol of certain human traits, a useful metaphor without ontological reality. Contemporary religious Satanism is predominantly an American phenomenon, although the rise of globalization and the Internet have seen these ideas spread to other parts of the world.

The term Satan has evolved from a Hebrew term for "adversary" or "to oppose", into the Christian figure of a fallen angel who tempts mortals into sin. The word Satan was not originally a proper name, but rather an ordinary noun that means "adversary". In this context, it appears at several points in the Old Testament. For instance, in the Book of Samuel, David is presented as the satan ("adversary") of the Philistines, while in the Book of Numbers, the term appears as a verb, when Jehovah sent an angel to satan ("to oppose") Balaam.

Prior to the composition of the New Testament, the idea developed within Jewish communities that Satan was the name of an angel who had rebelled against Jehovah and had been cast out of Heaven along with his followers; this account would be incorporated into contemporary texts such as the Book of Enoch. This Satan was then featured in parts of the New Testament, where he was presented as a figure who tempts humans to commit sin; in the Book of Matthew and the Book of Luke, he attempted to tempt Jesus of Nazareth as the latter fasted in the wilderness.

While the early Christian idea of the Devil was not well developed, it gradually adapted and expanded through the creation of folklore, art, theological treatises, and morality tales, thus providing the character with a range of extra-Biblical associations. Beginning in the early middle ages, the concept developed in Christianity of the devil as "archrepresentative of evil", and of the Satanist "as malign mirror image of the good Christian".

The word Satanism was adopted into English from the French satanisme. The terms Satanism and Satanist are first recorded as appearing in the English and French languages during the 16th century, when they were used by Christian groups to attack other, rival Christian groups. In a Roman Catholic tract of 1565, the author condemns the "heresies, blasphemies, and sathanismes [sic]" of the Protestants. In an Anglican work of 1559, Anabaptists and other Protestant sects are condemned as "swarmes of Satanistes [sic]". As used in this manner, the term Satanism was not used to claim that people literally worshipped Satan, but instead that they deviated from true Christianity, and thus were serving the will of Satan. During the 19th century, the term Satanism began to be used to describe those considered to lead a broadly immoral lifestyle, and it was only in the late 19th century that it came to be applied in English to individuals who were believed to consciously and deliberately venerate Satan. This latter meaning had appeared earlier in the Swedish language; the Lutheran Bishop Laurentius Paulinus Gothus had described devil-worshipping sorcerers as Sathanister in his Ethica Christiana, produced between 1615 and 1630.