Ellesmeroceras is the type genus for the Ellesmeroceratidae, a family of primitive nautiloid cephalopods, that is characterized by its small, generally compressed, gradually expanded, orthoconic shell, found in Lower Ordovician marine sediments.

The septa are close spaced and the siphuncle is ventral, about 0.2 the diameter of the shell.

Septal necks are typically orthochoanitic (short, straight) but may slant inwardly (loxochoanitic) or reach halfway to the previous septum (hemichoanitic).

Ellesmeroceras is one of three straight shelled Ellesmeroceratids, the other two being Ectenolites and Eremoceras.

It differs from Ectenolites, from which it is probably derived, in being stouter and proportionally wider, and from Eremoceras in being more straight overall.