Phacopida is an extinct order of trilobites known from the Early Ordovician to the end of the Devonian. They are most readily recognized by their unique schizochroal eyes, composed of large, separate lenses with individual corneal coverings - unlike the fused, densely packed lenses of the more common holochroal type found in most other trilobites. The cephalon (head) often features a prominent, inflated glabella and distinct facial sutures that facilitated molting. Many phacopids were capable of enrolling into a tight ball, a defensive adaptation.

These features set them apart from other trilobite orders such as Redlichiida or Corynexochida, which typically lack schizochroal eyes and show more primitive cephalic structures. Phacopids also tend to have more elaborate thoracic and pygidial morphology.

Fossils of Phacopida are widespread, especially in what are now North America, Europe, North Africa, and parts of Asia and South America, reflecting their success in diverse shallow marine environments before their extinction in the Late Devonian.