The Duke of Slavonia (Croatian: slavonski herceg; Latin: dux Slavoniae), also meaning the Duke of Dalmatia and Croatia (Croatian: herceg Hrvatske i Dalmacije; Latin: dux Dalmatiae et Croatiae) was a title of nobility granted several times in the 12th and 14th centuries, mainly to relatives of Hungarian monarchs or other noblemen.
The title of duke of "whole of Slavonia" didn't mean Slavonia in the narrow sense, but specifically all Slavic lands of the Kingdom of Hungary, being a synonym for the Kingdom of Croatia and Dalmatia (and Slavonian domain). An example being 1231 charter by Coloman in which protected "omnes templarios, qui infra ducatum Sclavonie sunt, tam in Dalmatia, quam in Croatia" ("all the Templars who are within the Duchy of Sclavonia, both in Dalmatia and in Croatia"). The title of duke signified a more extensive power than that of the Ban of Slavonia or Ban of Croatia. In cca. 1185 during Hungarian king Béla III, the "dux Sclauonie" paid to the king each year ten thousand silver coins.