Selonian was an East Baltic language, which was spoken by the East Baltic tribe of the Selonians, who until the 15th century lived in Selonia, a territory in southeastern Latvia and northeastern Lithuania.

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 northeastern sub-dialects of the Aukštaitian dialect of the Lithuanian language, mostly in the lexicon.

It is considered that the Selonian language retained the Proto-Baltic sonorant diphthongs *an, *en, *in, *un like the Lithuanian language, but like the Latvian language the Proto-Baltic *kʲ, *ɡʲ changed to c, dz, and the Proto-Baltic *š, *ž changed to s, z.^[1]