Reduta Palace in Cluj-Napoca, the building where the Diet of Transylvania functioned after 1790

The Transylvanian Diet (German: Siebenbürgische Landtag; Hungarian: Erdélyi Dieta; Romanian: Dieta Transilvaniei) was the constitutional and political body of the Principality of Transylvania, and later of the Grand Principality of Transylvania. It consisted of the three privileged nations (the Hungarians, the Székelys, and the Transylvanian Saxons) belonging to the received religions (Roman Catholic, Lutheran, Reformed and later Unitarian).

The diet sessions at Vásárhely (now Târgu Mureş) (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.