Duchy of Lithuania (Latin: Ducatus Lithuaniae; Lithuanian: Lietuvos kunigaikštystė) was a state-territorial formation of ethnic Lithuanians, that existed from the 13th century until 1413. Most of the time it was a constituent part and a nucleus of the Grand Duchy of Lithuania. Other alternative names of the territorial formation, used in different periods, were Aukštaitija or Land of Lithuania (13th century), Duchy of Vilnius (14th – early 15th centuries),Lithuania Propria or simply Lithuania (in a narrow sense).

The formation emerged in the central and eastern part of the present-day Lithuania, known as Aukštaitija, as the Lietuva Land (Lithuanian: Lietuvos žemė). It is supposed to have formed in the central Lithuania on the left bank of the Neris River and swiftly expanded eastwards. This very land was mentioned in 1009 as Litua (see Name of Lithuania). The territory was ruled by chieftains of an ethnic Lithuanian tribe, Aukštaitians or "Lithuanians", in the initial meaning of the name.

After the expansion of the Lithuanian state in the 13th century, when it became known as the Grand Duchy of Lithuania and the Kingdom of Lithuania (1251–1263), the Duchy of Lithuania became an administrative unit, governed by dukes and inherited by dynastic links. The main administrative center of the Duchy until the late 13th century might have been Kernavė.

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