Sindarin is one of the constructed languages devised by J. R. R. Tolkien for use in his fantasy stories set in Arda, primarily in Middle-earth. Sindarin is one of the many languages spoken by the Elves.

The word Sindarin is Quenya for Grey-elven, since it was the language of the Grey Elves of Beleriand. These were Elves of the Third Clan who remained behind in Beleriand after the Great Journey. Their language became estranged from that of their kin who sailed over the sea. Sindarin derives from an earlier language called Common Telerin, which evolved from Common Eldarin, the tongue of the Eldar before their divisions, e.g., those Elves who decided to follow the Vala Oromë and undertook the Great March to Valinor. Even before that the Eldar Elves spoke the original speech of all Elves, or Primitive Quendian.

In the Third Age (the setting of The Lord of the Rings), Sindarin was the language most commonly spoken by most Elves in the Western part of Middle-earth. Sindarin is the language usually referred to as the Elf-Tongue or Elven-Tongue in The Lord of the Rings. When the Quenya-speaking Noldor returned to Middle-earth, they adopted the Sindarin language. Quenya and Sindarin were related, with many cognate words but differing greatly in grammar and structure. Sindarin is said to be more changeful than Quenya, and there were during the First Age a number of regional dialects. The tongue used in Doriath (home of Thingol, King of the Sindar), known as Doriathrin, was said by many Grey-elves to be the highest and most noble form of the language.

In the Second Age, many Men of Númenor spoke Sindarin fluently. Their descendants, the Dúnedain of Gondor and Arnor, continued to speak Sindarin in the Third Age. Sindarin was first written using the Cirth, an Elvish runic alphabet. Later, it was usually written in the Tengwar (Quenya for 'letters') – a script invented by the elf Fëanor. Tolkien based the phonology and some of the grammar of Sindarin on Literary Welsh, and Sindarin displays some of the consonant mutations that characterize the Celtic languages.

The Dwarves rarely taught their language to others, so they learned both Quenya and Sindarin in order to communicate with the Elves, especially the Noldor and Sindar. By the Third Age, however, the Dwarves were estranged from the Elves and no longer routinely learned their language, preferring to use Westron.

For Tolkien's constructed languages one must distinguish two timelines of development:

Tolkien was interested in languages from an early age, and developed several constructed languages while still a teenager. Eventually, as a young adult, he created an entire family of constructed languages spoken by Elves and a secondary world where these could evolve.

One of these languages was created around 1915, inspired by the Celtic languages, particularly Literary Welsh. Tolkien called it Goldogrin or "Gnomish" in English. He wrote a substantial dictionary of Gnomish and a grammar. This is the first conceptual stage of the Sindarin language. At the same time Tolkien conceived a History of the Elves and wrote it in the Book of Lost Tales. Gnomish was spoken by the Gnomes or Noldoli, the Second Clan of Elves, and Elfin was the other tongue spoken by the great majority of the Elves of the Lonely Isle.