Orthoceras ("straight horn") is a genus of extinct nautiloid cephalopod. This genus is sometimes called Orthoceratites. Note it is sometimes misspelled as Orthocera, Orthocerus or Orthoceros.

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. Many other very similar species are included under the genus Michelinoceras. Orthoceras's closest living relative is the Nautilu.

Originally Orthoceras referred to all nautiloids with a straight-shell , called an ("orthocone"). But later research on their internal structures, such siphuncle, cameral deposits and others, showed that these actually belong to a number of groups, even different orders.