The Diet of Worms of 1521 (German: Reichstag zu Worms [astak tsu vms]) was an imperial diet (a formal deliberative assembly) of the Holy Roman Empire called by Emperor Charles V and conducted in the Imperial Free City of Worms. Martin Luther was summoned to the Diet in order to renounce or reaffirm his views in response to a Papal bull of Pope Leo X. In answer to questioning, he defended these views and refused to recant them. At the end of the Diet, the Emperor issued the Edict of Worms (Wormser Edikt), a decree which condemned Luther as "a notorious heretic" and banned citizens of the Empire from propagating his ideas. Although the Protestant Reformation is usually considered to have begun in 1517, the edict signals the first overt schism.

The diet was conducted from 28 January to 25 May 1521 at the Heylshof Garden, with the Emperor presiding. Other imperial diets took place at Worms in the years 829, 926, 1076, 1122, 1495, and 1545, but unless plainly qualified, the term "Diet of Worms" usually refers to the assembly of 1521.

Martin Luther (; German: [ˈmaʁtiːn ˈlʊtɐ] (listen); 10 November 1483 – 18 February 1546) was a German priest, theologian, author and hymnwriter. A former Augustinian friar, he is best known as the seminal figure in the Protestant Reformation and as the namesake of Lutheranism.

Luther was ordained to the priesthood in 1507. He came to reject several teachings and practices of the Roman Catholic Church; in particular, he disputed the view on indulgences. Luther proposed an academic discussion of the practice and efficacy of indulgences in his Ninety-five Theses of 1517. His refusal to renounce all of his writings at the demand of Pope Leo X in 1520 and the Holy Roman Emperor Charles V at the Diet of Worms in 1521 resulted in his excommunication by the pope and condemnation as an outlaw by the Holy Roman Emperor.

Luther taught that salvation and, consequently, eternal life are not earned by good deeds but are received only as the free gift of God's grace through the believer's faith in Jesus Christ as redeemer from sin. His theology challenged the authority and office of the pope by teaching that the Bible is the only source of divinely revealed knowledge, and opposed sacerdotalism by considering all baptized Christians to be a holy priesthood. Those who identify with these, and all of Luther's wider teachings, are called Lutherans, though Luther insisted on Christian or Evangelical (German: evangelisch) as the only acceptable names for individuals who professed Christ.

His translation of the Bible into the German vernacular (instead of Latin) made it more accessible to the laity, an event that had a tremendous impact on both the church and German culture. It fostered the development of a standard version of the German language, added several principles to the art of translation, and influenced the writing of an English translation, the Tyndale Bible. His hymns influenced the development of singing in Protestant churches. His marriage to Katharina von Bora, a former nun, set a model for the practice of clerical marriage, allowing Protestant clergy to marry.In two of his later works, Luther expressed antagonistic, violent views towards Jews and called for the burnings of their synagogues and their expulsion. His rhetoric was not directed at Jews alone but also towards Roman Catholics, Anabaptists, and nontrinitarian Christians. Luther died in 1546 with Pope Leo X's excommunication still in effect.