Diademoceras is a genus of nautiloid cephalopods from the middle Devonian of North America, named by Rousseau Flower in 1945.

Whorls are broad, section depressed, sides converge ventrally meeting at a well developed keel.

Ornamentation consists of a series of spines on either side as well as transverse striae which form a well developed hyponomic sinus (for the funnel) on the venter.

Sutures are straight across the venter, but form a broad shallow lobe on the dorsum.

The siphuncle is close to the venter, composed of straight and empty segments, i.e. orthochoanitic.