Mesolenellus is an extinct genus of trilobites that lived during the lower Cambrian (Botomian), found in Greenland and Spitsbergen.

[5] Mesolenellus hyperborea occurs together with Limniphacos perspiculum, Serrodiscus, hyoliths, Petrianna fulmenta (Bradoriida), and inarticulate brachiopods.

Like all other members of the superfamily Olenelloidea, the eye-ridges spring from the back of the frontal lobe (L4) of the central area of the cephalon, that is called glabella.

Mesolenellus also shares the typical character of the whole family Olenellidae that the frontal (L3) and middle pair (L2) of lateral lobes of the glabella are partially merged.

Mesolenellus can be distinguished from the sister-genus Mesonacis, where the back of the eye-ridge extends only to the most backward side lobes (L1), genal spines are only 1-5 times as long as L0, and the glabella widens forwards along L1 and L2.