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.