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

It has been collected from the Lower Cambrian (early Botomian) of Canada (Newfoundland), Russia (Siberia, Gorno-Altaysk), and the United States (New York State).

Like all Weymouthiidae, Semadiscus lacks eyes and rupture lines (or sutures).

It carries a stout backward-directed spine that is as long and one eighth as wide as the glabella.

The articulate middle part of the body (or thorax) and tailshield (or pygidium) are unknown.