Lutheranism or Evangelical Lutheranism is a major branch of Protestantism that emerged under the work of Martin Luther, the 16th-century German friar and reformer whose efforts to reform the theology and practices of the Catholic Church launched the Reformation in 1517. The Lutheran Churches adhere to the Bible and the Ecumenical Creeds, with Lutheran doctrine being explicated in the Book of Concord. Lutherans hold themselves to be in continuity with the apostolic church and affirm the writings of the Church Fathers and the first four ecumenical councils.

The schism between Roman Catholicism and Lutheranism, which was formalized in the Edict of Worms of 1521, centered on two points: the proper source of authority in the church, often called the formal principle of the Reformation, and the doctrine of justification, the material principle of Lutheran theology. Lutheranism advocates a doctrine of justification "by Grace alone through faith alone on the basis of Scripture alone", the doctrine that scripture is the final authority on all matters of faith. This contrasts with the belief of the Roman Catholic Church, defined at the Council of Trent, which contends that final authority comes from both Scripture and tradition. In Lutheranism, tradition is subordinate to Scripture and is cherished for its role in the proclamation of the Gospel.

The Lutheran Churches retain many of the liturgical practices and sacramental teachings of the pre-Reformation Western Church, with a particular emphasis on the Eucharist, or Lord's Supper, although Eastern Lutheranism uses the Byzantine Rite. Though Lutherans are not dogmatic about the number of sacraments, three Lutheran sacraments are generally recognized including baptism, confession and the eucharist. The Lutheran Churches teach baptismal regeneration, that humans "are cleansed of our sins and born again and renewed in Holy Baptism by the Holy Ghost". Lutheranism teaches that sanctification commences at the time of justification and that Christians, as a result of their living faith, ought to do good works, which are rewarded by God. The act of mortal sin forfeits salvation, unless individuals turn back to God through faith. In the Lutheran Churches, the Office of the Keys exercised through confession and absolution is the "authority which Christ has given to His Church on earth: to forgive the sins of the penitent sinners, but to retain the sins of the impenitent as long as they do not repent." The doctrine of the real presence of Christ in the Eucharist via a sacramental union is central to the Lutheran faith, with the Mass (also known as the Divine Service) being celebrated regularly, especially on the Lord's Day.

Lutheranism became the state church of many parts of Northern Europe, starting with Prussia in 1525. In Scandinavia, the Roman Catholic bishops largely accepted the Lutheran reforms and the Church there became Lutheran in belief; the threefold ministry of bishops, priests, and deacons was continued. Lutheran divines who contributed to the development of Lutheran theology include Martin Luther, Martin Chemnitz, Philip Melanchthon, Joachim Westphal, Laurentius Petri, Olaus Petri, and Laurentius Andreae.

Lutheranism has contributed to Christian hymnody and the arts, as well as the development of education. Christian missions have been established by Lutherans in various regions. Lutheran Churches operate a number of Lutheran schools, colleges and universities around the world, in addition to hospitals and orphanages. A number of Lutheran religious orders, as well as monasteries and convents, live in community to pray and work. Lutherans are found across all continents of the globe, numbering 90 million.

The name Lutheran originated as a derogatory term used against Luther by German Scholastic theologian Johann Maier von Eck during the Leipzig Debate in July 1519. Eck and other Roman Catholics followed the traditional practice of naming a heresy after its leader, thus labeling all who identified with the theology of Martin Luther as Lutherans.

Evangelical Lutheranism derives from the word "evangelical" (derived from εὐαγγέλιον euangelion, a Greek word meaning "good news", i.e. "Gospel") and "Lutheran" in reference to Martin Luther, who contributed to the development of the tradition. Martin Luther always disliked the term Lutheran, preferring the term evangelical. The Evangelical Lutheran Catechism notes that the Church uses the term "Evangelical" to show "her adherence to the pure evangelical or gospel doctrines, and to the Scriptures in general". The followers of John Calvin, Huldrych Zwingli, and other theologians linked to the Reformed tradition also used that term. To distinguish the two evangelical groups, others began to refer to the two groups as Evangelical Lutheran and Evangelical Reformed. As time passed by, the word Evangelical was not always used; Lutherans themselves began to use the term Lutheran in the middle of the 16th century, in order to distinguish themselves from other groups such as the Anabaptists and the Reformed (Calvinist) tradition. A large number of Evangelical-Lutheran denominations use the full form, such as the Evangelical Lutheran Church in Kenya, Wisconsin Evangelical Lutheran Synod, and Evangelical Lutheran Church of Finland. That being said, many personal opinions of Martin Luther were not adopted by the Lutheran Churches in the Augsburg Confession (the primary confession of faith of the Lutheran Church), and because Lutheranism retained much of the pre-Reformation liturgical and devotional practices, the Lutheran Reformation is generally considered to be the most conservative among the Protestant traditions.

In various parts of the world, the terms "Evangelical Lutheranism", "Evangelicalism", and "Lutheranism" are used to refer to the faith.