Sphaeragnostus is an extinct genus from a well-known class of fossil marine arthropods, the trilobites.

It can be recognized by having two thorax segments, a totally effaced headshield (or cephalon), while the tailshield (or pygidium) although effaced, has a clear furrow parallel to its border, and a short, convex, subcircular axis.

[2] It is difficult to relate Sphaeragnostus to other agnostids, and it is undecided whether this genus should be assigned to the Agnostoidea, the Condylopygoidea, or a new monotypic superfamily.

[1] Like all Agnostida, Sphaeragnostus is diminutive and the cephalon and pygidium are of approximately the same size and outline (or isopygious).

Pygidia of Sphaeragnostus however, differ from other highly effaced genera in a smooth but easily recognizable subcircular axis.