The Lithuanian nobility (Lithuanian: bajorija) or szlachta of the Grand Duchy of Lithuania (Lithuanian: Lietuvos Didžiosios Kunigaikštystės šlėkta, Polish: szlachta Wielkiego Księstwa Litewskiego) was historically a legally privileged hereditary elite class in the Grand Duchy of Lithuania and Polish Lithuanian Commonwealth (including during period of foreign rule 1795–1918) consisting of Lithuanians from Lithuania Proper; Samogitians from Duchy of Samogitia; following Lithuania's eastward expansion into what is now Belarus, Ukraine and Russia, many ethnically Ruthenian noble families (boyars); and, later on, predominantly Baltic German families from the Duchy of Livonia and Inflanty Voivodeship.
Initially, the privileged social group of the Grand Duchy of Lithuania was called boyars. Boyars became part of the szlachta (nobility) during the Union of Horodło on October 2, 1413, initiating nobility in the Grand Duchy of Lithuania following the Western European model (with a hereditary system of heraldic identification), as well as an increase in the position of the Greater Lithuanian nobility. The Grand Duchy of Lithuania adopted Polish institutions of castellans and voivodes, and 47 selected boyars of Grand Duchy of Lithuania of the Catholic faith were adopted by Polish noble families and received Polish coats of arms.