Anabaptism (from Neo-Latin anabaptista, from the Greek ἀναβαπτισμός: ἀνά 're-' and βαπτισμός 'baptism'; German: Täufer, earlier also Wiedertäufer) is a Christian movement which traces its origins to the Radical Reformation in the 16th century. Anabaptists believe that baptism is valid only when candidates freely confess their faith in Christ and request to be baptized. This stance, commonly referred to as believer's baptism, is opposed to the baptism of infants, who are not able to make a conscious decision to be baptized.

The early Anabaptists formulated their beliefs in a confession of faith in 1527 called the Schleitheim Confession. Its author Michael Sattler was arrested and executed shortly afterward. Anabaptist groups varied widely in their specific beliefs, but the Schleitheim Confession represents foundational Anabaptist beliefs as well as any single document can.

Other Christian groups with different roots also practice believer's baptism, such as Baptists, but these groups are not Anabaptist, even though the Baptist tradition was influenced by the Anabaptist view of Baptism. The Amish, Hutterites and Mennonites are direct descendants of the early Anabaptist movement. Schwarzenau Brethren, River Brethren, Bruderhof and the Apostolic Christian Church are Anabaptist denominations that developed after the Radical Reformation, following their example. Though all Anabaptists share the same core theological beliefs, there are differences in the way of life among them; Old Order Anabaptist groups include the Old Order Amish, the Old Order Mennonites, Old Order River Brethren and the Old Order German Baptist Brethren. In between the assimilated mainline denominations (such as Mennonite Church USA and the Church of the Brethren) and Old Order groups are Conservative Anabaptist groups. Conservative Anabaptists such as the Dunkard Brethren Church, Conservative Mennonites and Beachy Amish have retained traditional religious practices and theology, while allowing for judicious use of modern conveniences and advanced technology.

Emphasizing an adherence to the beliefs of early Christianity, Anabaptists in general are distinguished by their keeping of practices that often include nonconformity to the world: "the love feast with feet washing, laying on of hands, anointing with oil, and the holy kiss, as well as turning the other cheek, no oaths, going the second mile, giving a cup of cold water, reconciliation, repeated forgiveness, humility, non-violence, and sharing possessions."

The name Anabaptist originated as an exonym meaning "one who baptizes again", referring to the practice of baptizing persons when they converted or declared their faith in Christ, when they had already been baptized as infants. Many called themselves "Radical Reformers". Anabaptists require that baptismal candidates be able to make a confession of faith that is freely chosen. They understand the New Testament order is to repent and then be baptized, and infants are unable to repent and turn away from sin to a life of following Jesus; thus infant baptism is invalid. The early members of this movement did not accept the name Anabaptist, claiming that infant baptism was not part of scripture and was therefore null and void. They said that baptizing self-confessed believers was their first true baptism:

I have never taught Anabaptism. ...But the right baptism of Christ, which is preceded by teaching and oral confession of faith, I teach, and say that infant baptism is a robbery of the right baptism of Christ.

— Hubmaier, Balthasar (1526), Short apology.

Anabaptists were heavily persecuted by state churches, both Magisterial Protestants and Roman Catholics, beginning in the 16th century and continuing thereafter, largely because of their interpretation of scripture which put them at odds with official state church interpretations and local government control. Anabaptism was never established by any state and therefore never enjoyed any associated privileges. Most Anabaptists adhere to a literal interpretation of the Sermon on the Mount in Matthew 5–7, which teaches against hate, killing, violence, taking oaths, participating in use of force or any military actions, and against participation in civil government. Anabaptists view themselves as primarily citizens of the kingdom of God, not of earthly nations. As committed followers of Jesus, they seek to pattern their life after his.