The Duchy of Samogitia had been the administrative unit of the Grand Duchy of Lithuania from 1422 (and from 1569, a part of the Polish-Lithuanian Commonwealth). Between 1422 to 1441 it was known as the Eldership of Samogitia. The Grand Duke of Lithuania also held the title of Duke of Samogitia, although the actual ruler of the province, responsible to the Duke was known as the General Elder (Seniūnas) of Samogitia.
The Duchy was located in the western part of the present Republic of Lithuania. Historically, its western border had been a Lithuanian border with the Baltic Sea; in the north, it bordered the Duchy of Courland and Ducal Prussia in the south. During the Middle Ages and until the last partition in 1795, Samogitia had clearly defined borders as the Duchy of Samogitia. Afterwards the area encompassed the Samogitian Diocese. Today Samogitia is one of several ethnographic regions and is not defined administratively.