大公国（拉丁语：Magnus Dux；德语：Großherzogtum；意大利语：Gran Duca；法语：Grand-duché；立陶宛语：Didysis kunigaikštis；波兰语：Wielkie księstwo；英语：Grand duchy），是一种君主制国家，其元首是由大公或者是女大公担任。

而相近的就是公国（Dukedom）和亲王国（Principality，意译是「亲王的封地」），虽然名称有别，而且国君的头衔上，亲王最大，大公次之，公爵更次之；但实际上都是平等论交的君主，而不是字面上的同一国的不同等级王族或贵族。

A grand duchy is a country or territory whose official head of state or ruler is a monarch bearing the title of grand duke or grand duchess. Relatively rare until the abolition of the Holy Roman Empire in 1806, the term was often used in the official name of countries smaller than most continental kingdoms of modern Europe (e.g., Denmark, Spain, United Kingdom) yet larger than most of the sovereign duchies in the Holy Roman Empire, Italy or Scandinavia (e.g. Anhalt, Lorraine, Modena, Schleswig-Holstein). During the 19th century there were as many as 14 grand duchies in Europe at once (a few of which were first created as exclaves of the Napoleonic empire but later re-created, usually with different borders, under another dynasty). Some of these were sovereign and nominally independent (Baden, Hesse and by Rhine, Mecklenburg-Schwerin, Mecklenburg-Strelitz, Oldenburg, Saxe-Weimar and Tuscany), some sovereign but held in personal union with larger realms by a monarch whose grand-dukedom was borne as a subsidiary title (Finland, Luxembourg, Transylvania), some of which were client states of a more powerful realm (Cleves and Berg), and some whose territorial boundaries were nominal and the position purely titular (Frankfurt).