Falathrin is an extinct dialect of the conlang Sindarin.

Within the fiction of J. R. R. Tolkien's the Silmarillion, the Sindar of Beleriand were divided in several groups, and their language had several dialects. Falathrin, the flavour of Sindarin spoken by the Sindar of the Falas, the coastal regions of Beleriand, was the mainstream southern variant of Sindarin (the other being Doriathrin). Falathrin and Doriathrin were mutually intelligible without problems.

Spoken by the followers of Círdan the Shipwright, Falathrin was also adopted by the Ñoldorin king Finrod Felagund when he removed to Nargothrond, partially to make sure that his people would speak a slightly different language than the followers of the Sons of Fëanor, who spoke a Quendarized form of North Sindarin. As Finrod had close family relations to king Thingol of Doriath and had more contact with Thingol and Círdan than with the Fëanorians, it was an economic and political advantage to speak a more common form of Sindarin. During this time Falathrin was changed much, partially due to the adoption of Quenya features, and partially due to the love of the Ñoldor for making linguistic changes.

Falathrin is the direct ancestor of the Sindarin of the Third Age, as this was the base of the language which was adopted at Arvernien by the last survivors of Beleriand. Although altered some by contact with other languages, it was this form of Sindarin which was studied in Númenor and the later Númenorean realms in exile. As such, it stood at the base of Westron.