Illaenus is a genus of trilobites from Russia and Morocco, from the middle Ordovician.

Species included in this genus can reach a length of about 57 millimetres (2.2 in).

The cephalon has a high profile and recurved genal spines.

Eyes are distant from the axis of the head, situated nearer to the edge.

[1][2] This genus includes about 50 species:[3]