Selenosteus brevis is an extinct large selenosteid arthrodire placoderm known from the Famennian Cleveland Shale of Ohio.

Scrappy remains from the Frasnian Rhinestreet Shales of Erie County, New York, were attributed by Hussakof and Bryant to this genus in 1919, but, this identification is doubtful.

[1] According to its generally scrappy fossils, S. brevis had a wide skull with tremendous orbits.

And as typical for selenosteids, S. brevis had weak gnathal plates.

The average length of the skull is about 16 centimetres from the tip of the rostrum to the posterior border of the nuchal plate.