Orthoceras is a genus of extinct nautiloid cephalopod restricted to Middle Ordovician-aged marine limestones of the Baltic States and Sweden.

Orthoceras was formerly thought to have had a worldwide distribution due to the genus' use as a wastebasket taxon for numerous species of conical-shelled nautiloids throughout the Paleozoic and Triassic.

[1] These are slender, elongate shells with the middle of the body chamber transversely constricted, and a subcentral orthochoanitic siphuncle.

According to the authoritative Treatise on Invertebrate Paleontology, the name Orthoceras is now only used to refer to the type species O. regulare (Schlotheim 1820) harvcol error: no target: CITEREFSchlotheim1820 (help) from the Middle Ordovician of Sweden and parts of the former Soviet Union such as Russia, Ukraine, Belarus, Estonia and Lithuania.

The two types of fossils can be distinguished by many features, most obvious among which is the suture line: simple in Orthoceras (see image), intricately foliated in Baculites and related forms.