East Baltic language most closely related to Latvian; it is spoken primarily in Lithuania, where it has been the official language since 1918. It is the most archaic Indo-European language still spoken.

A Lithuanian literary language has been in existence since the 16th century, the earliest document being translations of the Lord's Prayer, a creed, and the Ave Maria, made about 1525. This language, used solely for writings of a religious character, differs in many respects from modern Lithuanian; thus, it has longer forms of some grammatical endings than does modern literary Lithuanian, has two more cases, shows stronger influences from Slavic in its vocabulary and syntax, and differs from the modern standard language in accentuation. Most of these characteristics survived through the first half of the 17th century.

Three literary dialects were in use in the 19th century: a Low Lithuanian dialect along the Baltic Sea coast, an East High Lithuanian poetic dialect, and a West High Lithuanian dialect, used primarily in the region bordering East Prussia. The modern standard literary language, written in a 32-letter Latin alphabet, is based on the West High Lithuanian dialect of the scholar Jonas Jablonskis (1861–1930), who is considered to be its father.

Like all Baltic languages, Lithuanian has preserved many archaic features from the ancestral Proto-Indo-European language; among these are the use of forms for the dual number in both nouns and verbs and, in Old Lithuanian, the locative plural ending -su.