The Lithuanian Metrica or the Metrica of the Grand Duchy of Lithuania (Latin: Acta Magni Ducatus Lithuaniae, Lithuanian: Lietuvos Metrika, Polish: Metryka Litewska, or Metryka Wielkiego Księstwa Litewskiego; Belarusian: Літоўская Метрыка, Ukrainian: Литовська метрика) is a collection of the 14–18th century legal documents of the Chancellery of the Grand Duchy of Lithuania (GDL). Maintained systematically since the 2nd half the 15th century, metrica consisted, initially and primarily, of the copies of the documents issued by the Grand Duke, Lithuanian Council of Lords, and Seimas.

The Metrica also included some important externally originated documents (like translations of the issues (yarlyks) of the Crimea Khans, copies of the Muscovy diplomatic documents etc.), the office-keeping documental materials (like registers of acts, inventories of the Metrica itself etc.) The selection of the classes of the documents included in the Metrica had increased since the 2nd half 15th – 16th century and even more so in the 17–18th centuries, extending to include the copies of transcripts of diplomatic correspondence, charters of privileges, wills, verdicts, judicial decrees, even certain kinds of private correspondence, e.g., received by the official persons. Sometimes, the external and thematically not quite related collections of the documents had also been referred to as the part of the Metrica, the word Metrica here to be understood as the State Archive.[1] The documents of the Metrica were to be preserved interminably.