Harpetida is one of the eleven orders of the extinct arthropod class Trilobita.

The first harpetid trilobites appear in the Upper Cambrian, and the last species die out at the end of the Devonian period.

The brim stretches backward on either side of the cephalon (head) and typically has a pronounced suture along the outside.

The compound eyes are typically reduced to small tubercles, though they have strong ridges stretching to the glabella (central region of the cephalon).

The subclass Librostoma was erected in 1990 by Richard Fortey to cover the various orders originally placed within Ptychopariida.