Acmarhachis is a genus of trilobites in the order Agnostida, which lived in what are now Australia (Queensland, Tasmania), Canada (British Columbia, Newfoundland, Northwest Territories), China (Anhui), Kazakhstan, Russia (Kharaulakh), and the US (Alaska, Alabama, Nevada, Maryland, Vermont).

[3] Like all Agnostida, Acmarhachis is diminutive and the headshield (or cephalon) and tailshield (or pygidium) are of approximately the same size (or isopygous) and outline.

When Agnostina are enrolled, a tiny opening between the cephalon and the thorax becomes visible (the cephalothoracic aperture) that uniquely distinguishes this group from the Eodiscina.

The glabella has a long ogival to subquadrate front lobe, and the furrow defining it at its back is straight.

It is shaped a bit like an amphora, the back pointed and strongly constricted across the middle lobe.