Arianism (Koine Greek: Ἀρειανισμός, Areianismós) is a Christological doctrine that rejects the traditional notion of the Trinity, teaching that Jesus was created by God and is therefore distinct from God. It is named after its proponent Arius (250 or 256 – 336) and is regarded as heretical by most modern mainstream branches of Christianity. Arianism is held by a minority of modern denominations, although some of these groups espouse related doctrines such as Socinianism, and others avoid the term "Arian" because of its historically negative connotations. Modern denominations sometimes associated with the teaching include Jehovah's Witnesses, some churches within the Churches of Christ (among them the movement's founder, Barton W. Stone), and certain Hebrew Roots Christians and Messianic Jews, although many Messianic Jews adhere to Nicene Christianity.

It is first attributed to Arius, a Christian presbyter who preached and studied in Alexandria, Egypt, although it developed out of various preexisting strands of Christianity which differed from later Nicene Christianity in their view of Christology. Arian theology holds that Jesus Christ is the Son of God, who was begotten by God the Father with the difference that the Son of God did not always exist but was begotten/made before time by God the Father; therefore, Jesus was not coeternal with God the Father, but nonetheless Jesus began to exist outside time.

Arius's trinitarian theology, later given an extreme form by Aetius and his disciple Eunomius and called anomoean ('dissimilar'), asserts a total dissimilarity between the Son and the Father. Arianism holds that the Son is distinct from the Father and therefore subordinate to him. The term Arian is derived from the name Arius; it was not what the followers of Arius's teachings called themselves, but rather a term used by outsiders. The nature of Arius's and his supporters' teachings were opposed to the theological doctrines held by Homoousian Christians regarding the nature of the Trinity and the nature of Christ. Homoousianism and Arianism were contending interpretations of Jesus's divinity, both based upon the trinitarian theological orthodoxy of the time.

Homoousianism was formally affirmed by the first two ecumenical councils; since then, Arianism has been condemned as "the heresy or sect of Arius". Trinitarian (Homoousian) doctrines were vigorously upheld by Patriarch Athanasius of Alexandria, who insisted that Jesus (God the Son) was "same in being" or "same in essence" with God the Father. Arius dissented: "If the Father begat the Son, then he who was begotten had a beginning in existence, and from this it follows there was a time when the Son was not." The ecumenical First Council of Nicaea of 325 declared Arianism to be a heresy. According to Everett Ferguson, "The great majority of Christians had no clear views about the nature of the Trinity and they did not understand what was at stake in the issues that surrounded it."

Arianism is also used to refer to other nontrinitarian theological systems of the 4th century, which regarded Jesus Christ—the Son of God, the Logos—as either a begotten creature of a similar or different substance to that of the Father, but not identical (as Homoiousian and Anomoeanism) or as neither uncreated nor created in the sense other beings are created (as in semi-Arianism).

Some early Christians whose beliefs would have fallen under 'orthodoxy' in the third and fourth centuries denied the eternal generation of the Son; they viewed the Son as having been begotten in time. These include Tertullian and Justin Martyr. Tertullian is considered a pre-Arian. Among the other church fathers, Origen was accused of Arianism for using terms like "second God", and Patriarch Dionysius of Alexandria was denounced at Rome for saying that Son is a work and creature of God (i.e., a created being). However, the subordinationism of Origen is not identical to Arianism, and it has been generally viewed as closer to the Nicene-Constantinopolitan view of the Trinity.

Controversy over Arianism arose in the late 3rd century and persisted throughout most of the 4th century. It involved most church members—from simple believers, priests, and monks to bishops, emperors, and members of Rome's imperial family. Two Roman emperors, Constantius II and Valens, became Arians or semi-Arians, as did prominent Gothic, Vandal, and Lombard warlords both before and after the fall of the Western Roman Empire. The antipopes Felix II and Ursinus were Arian, and Pope Liberius was forced to sign the Arian Creed of Sirmium of 357—though the letter says he willingly agreed with Arianism. Such a deep controversy within the early Church during this period could not have materialized without significant historical influences providing a basis for the Arian doctrines.

Arius had been a pupil of Lucian of Antioch at Lucian's private academy in Antioch and inherited from him a modified form of the teachings of Paul of Samosata. Arius taught that God the Father and the Son of God did not always exist together eternally.