Stenagostus rhombeus is a relatively large species of click beetle, growing up to 20mm in length. It has an elongated orange / brown coloured body, with a darker crossline near the tip of the elytra (wing casings), and another crossline just above this, but pointing the opposite direction (towards the pronotum).
The legs and antennae of Stenagostus rhombeus are a uniformly orange / brown colour, and, like the rest of its body, covered in short white hairs.