Lithuanian (lietuvių kalba, pronounced [lʲiəˈtʊvʲuː kɐɫˈbɐ]) is an East Baltic language belonging to the Baltic branch of the Indo-European language family. It is the language of Lithuanians and the official language of Lithuania as well as one of the official languages of the European Union. There are approximately 2.8 million native Lithuanian speakers in Lithuania and about 1.5 million speakers elsewhere. Around half a million inhabitants of Lithuania of non-Lithuanian background speak Lithuanian daily as a second language.

Lithuanian is closely related to neighbouring Latvian, though the two languages are not mutually intelligible. It is written in a Latin script. Some linguists consider it to be the most conservative of the extant Indo-European languages, retaining features of the Proto-Indo-European language that have otherwise been lost to more recent linguistic developments in the remainder of its descendant languages.

Anyone wishing to hear how Indo-Europeans spoke should come and listen to a Lithuanian peasant.

Among Indo-European languages, Lithuanian is conservative in its grammar and phonology, retaining archaic features otherwise found only in ancient languages such as Sanskrit (particularly its early form, Vedic Sanskrit) or Ancient Greek. Thus, it is an important source for the reconstruction of the Proto-Indo-European language despite its late attestation (with the earliest texts dating only to c. 1500 AD, whereas Ancient Greek was first written down about three thousand years earlier in c. 1450 BC).

According to hydronyms of Baltic origin, the Baltic languages were spoken in a large area east of the Baltic Sea, and in c. 1000 BC it had two linguistic units: western and eastern. According to glottochronological research, the Eastern Baltic languages split from the Western Baltic ones between c. 400 BC and c. 600 BC. The Greek geographer Ptolemy had already written of two Baltic tribe/nations by name, the Galindai (Γαλίνδαι) and Sudinoi (Σουδινοί), in the 2nd century AD. Lithuanian originated from the Eastern Baltic subgroup and remained nearly unchanged until c. 1 AD, however in c. 500 AD the language of the northern part of Eastern Balts was influenced by the Finnic languages, which fueled the development of changes from the language of the Southern Balts (see: Latgalian, which developed into Latvian, and extinct Curonian, Semigallian, and Selonian). The language of Southern Balts was less influenced by this process and retained many of its older features, which form Lithuanian.

The differentiation between Lithuanian and Latvian started after c. 800 AD; for a long period, they could be considered dialects of a single language. At a minimum, transitional dialects existed until the 14th or 15th century and perhaps as late as the 17th century. The German Livonian Brothers of the Sword occupied the western part of the Daugava basin, which resulted in colonization of the territory of modern Latvia (at the time it was called Terra Mariana) by Germans and had a significant influence on the language's independent development due to Germanisation (see also: Baltic Germans and Baltic German nobility).

There was fascination with the Lithuanian people and their language among the late 19th-century researchers, and the philologist Isaac Taylor wrote the following in his The Origin of the Aryans (1892):

"Thus it would seem that the Lithuanians have the best claim to represent the primitive Aryan race, as their language exhibits fewer of those phonetic changes, and of those grammatical losses which are consequent on the acquirement of a foreign speech."