The Diet of Worms of 1521 (German: Reichstag zu Worms [ˈʁaɪçstaːk tsuː ˈvɔʁms]) 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.