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). They are without glabella and without articulation of the tail. 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]

• †Illaenus alveatus Raymond 1925
• †Illaenus auriculatus Ross Jr. & Barnes 1967
• †Illaenus bayfieldi Billings 1859
• †Illaenus bucculentus Whittington 1963
• †Illaenus consimilis Billings 1865
• †Illaenus consobrinus Billings 1865
• †Illaenus crassicauda Wahlenberg 1826
• †Illaenus fraternus Billings 1865
• †Illaenus gelasinus Whittington 1965
• †Illaenus lacertus Whittington 1954
• †Illaenus marginalis Raymond 1925
• †Illaenus oscitatus Fortey 1980
• †Illaenus slancyensis Krylov 2016
• †Illaenus spiculatus Whittington 1963
• †Illaenus tauricornis Kutorga 1848
• †Illaenus tumidifrons Billings 1865
• †Illaenus utahensis Hintze 1952
• †Illaenus welchi Loch & Ethington 2017

• Ordovician fauna of Putilovo village

• Yvonne Howells, "Ordovician Trilobites of St. Petersburg Region"

• Russian Trilobites
• Saint Petersburg Paleontological Laboratory Archived 2012-03-24 at the Wayback Machine