The Transylvanian Diet (German: Siebenbürgischer Landtag; Hungarian: erdélyi országgyűlés; Romanian: Dieta Transilvaniei) was an important legislative, administrative and judicial body of the Principality (from 1765 Grand Principality) of Transylvania between 1570 and 1867. The general assemblies of the Transylvanian noblemen and the joint assemblies of the representatives of the "Three Nations of Transylvania"—the noblemen, Székelys and Saxons—gave rise to its development. After the disintegration of the medieval Kingdom of Hungary in 1541, delegates from the counties of the eastern and northeastern territories of Hungary proper (or Partium) also attained the Transylvanian Diet, transforming it into a legal successor of the medieval Diets of Hungary.
The diet sessions at Vásárhely (now Târgu Mureș) (20 January 1542) and at Torda (now Turda) (2 March 1542) laid the basis for the political and administrative organization of Transylvania. The diet decided on juridical, military and economic matters. It ceased to exist following the Austro-Hungarian Compromise of 1867, when Transylvania was returned to the control of the Diet of the newly empowered Hungary.