The Lithuanian Metrica or the Metrica of the Grand Duchy of Lithuania (Latin: Acta Magni Ducatus Lithuaniae, Lithuanian: Lietuvos Metrika, or Lietuvos didžiosios kunigaikštystės 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 of 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. The documents of the Metrica were to be preserved interminably.

Effectively, the Metrica had become the core of the Archive of the Grand Duke, later the core of the Main State Archive of the GDL, serving the notifying (judicial-registrative), judicial, referential functions. It had been the source of the authoritative official documents (copies of copies). The Metrica developed parallel to and on the model of the Crown Metrica of Poland.

Today, over 600 (estimated) books of the Lithuanian Metrica still exist. Their microfilms are preserved at the Lithuanian State Historical Archives in Vilnius.

The word metrica generates from Polish: metryka for archive, from Latin: matricula for office book.

The first historical names of the collection were metrics, books of metrica, metrica. Since the end of the 16th – beginning of the 17th century, the full official name was Metrica of the Grand Duchy of Lithuania. This would be the most appropriate scientific name. The dub Lithuanian Metrica had been occasionally used in the local office books (by analogy with the Crown Metrica of Poland), since the mid-17th century the dub had consolidated its position in the documents of the Warsaw Archives, later in the Archives of Moscow and St. Petersburg, then in the 19–20th centuries Russian, Polish, Belarusian, Lithuanian historiographies. The name Lithuanian Metrica is till now traditionally used in the Western publications of the Metrica. The name Lithuanian Metrica is also used in reference to some contemporary archival collections, including some of the materials of the Chancellery of the GDL (together with other unrelated materials), chiefly in Russia custody.

The prevailing language of the documents of the 15th and most of the 16th century Metrica had been the Ruthenian language. Alternatively, the prevailing use of the Ruthenian language in the Metrica is extended up to the mid-17th century. The documents, concerned with the Western Europe, had been issued in Latin, occasionally in German. The documents, concerned with the Roman Catholic Church, had been issued in Latin. See also naming of the Ruthenian language.

Since the late 16th – early 17th century, the number of the documents composed in Polish and in Latin, had been steadily increasing, until the complete elimination of the Ruthenian from the office use in the GDL, and further official ban on the Ruthenian for the official use (1696). The language of the 17–18th century Metrica is mostly Polish and partly Latin.