Orthoceras ("straight horn") is a genus of extinct nautiloid cephalopod. This genus is sometimes called Orthoceratites.
Orthoceras fossils are common and have a global distribution, occurring in any marine rock, especially limestones.
These are slender, elongate shells with the middle of the body chamber transversely constricted, and a subcentral orthochoanitic siphuncle. The surface is ornamented by a network of fine lirae (Sweet 1964:K224). Many other very similar species are included under the genus Michelinoceras. Although it has an external, chambered shell like the Nautilus, Orthoceras is probably more closely related to coleoids.