Selonian was a language appertaining to the Baltic languages group of the Indo-European languages family. It was spoken by the Eastern Baltic tribe of the Selonians, who until the 15th century lived in Selonia, a territory in South Eastern Latvia and North Eastern Lithuania.

During the 13th-15th centuries the Selonians lost their language after being assimilated by the Latgalians and partly by the Lithuanians.

It is considered that the Selonian language retained the proto-Baltic phonemes *an, *en, *in, *un like the Lithuanian language, but like the Latvian language the proto-Baltic , changed to c, dz, and the proto Baltic *š, *ž changed to s, z.

Traces of the Selonian language can still be found in the territories the Selonians inhabited, especially in the accent and phonetics of the so-called Selonian dialect of the Latvian language.
There are some traces of the Selonian language in the North Eastern sub-dialects of the Aukštaitian dialect of Lithuanian language, mostly in the lexicon.