Doriathrin was the dialect of Ilkorin spoken by the Elves of Doriath (LR/175). In The Etymologies, Tolkien marked almost as many words as Doriathrin (Dor.) as he did Ilkorin (Ilk.). There are very few examples of words appearing in both dialects, however, and almost all of them have the same form. In fact, the only words with distinct Ilkorian and Doriathrin forms are the names Ilk. Argad/Argador (Ety/AR²) versus Dor. Argador (Ety/GAT(H)), and Ilk. Mablosgen (Ety/MAP) versus Dor. Mablost (Ety/KAB).

Despite the lack of evidence, the examples in The Etymologies do hint at several possible differences between the two dialects:

• There are a number of Ilkorin words for which final [u] became [o], but there are several Doriathrin words where this does not occur: Dor. ganu, Dor. gelu and Dor. hedhu. An exception is Dor. (n)golo which does seem to have this change.
• There are a couple of examples in which final [ð] became [θ] in Doriathrin, but this change did not occur in the Ilkorin words Ilk. erdh and Ilk. gwedh.
• There are no examples of Ilkorin i-affection in Doriathrin plural nouns even in cases where it should occur, whereas there are plenty examples of such changes in the plurals of Ilkorin nouns.
• According to one note (Ety/NAUK), the genitive generally preceded the modified noun in the Doriathrin dialect, such as Dor. Nauglamír. There are several counter-examples in Ilkorin such as Ilk. Tor Tinduma and Ilk. Torthurnion.

Like Falathrin, the Doriathrin dialect influenced the common language of the survivors of the wars of Beleriand spoken at the mouth of the Sirion river (LR/178).