Sinclairoceras is a nautiloid cephalopod in the discosorid family Westonoceratidae from the Middle Ordovician Simard Limestone of Quebec.
The shell is laterally compressed, upwardly curved, and short; an exogastric brevicone with the venter narrowly and dorsum broadly rounded.
The venter, at the lower surface, is more strongly convex longitudinally, drawing away from the dorsum and forming a hump at the anterior end of the phragmocone where Sinclairoceras reaches its greatest width.
The upper and lower sides of the body chamber are essentially straight and virtually parallel, flaring slightly at the aperture.
Sinclairoceras differs from the overall similar Westonoceras in being shorter and less markedly swollen at the mature end of the phragmocone.