The Duchy of Courland and Semigallia (, , , ) is the name of a duchy in the Baltic region that existed from 1561 to 1569 as a vassal state of the Grand Duchy of Lithuania and from 1569 to 1726 the Polish-Lithuanian Commonwealth, incorporated into Polish-Lithuanian Commonwealth by Sejm in 1726, but on 28 March 1795, it was annexed by the Russian Empire in the Third Partition of Poland.