Actinoceras is the principal and root genus of the Actinoceratidae, a major family in the Actinocerida, that lived during the Middle and Late Ordovician.

It is an extinct genus of nautiloid cephalopod that thrived in the warm waters of the United States and England during the Paleozoic era.

Actinoceras are generally large, with typically straight shells reaching a meter or so in length (about 3 ft), with a blunt apex, and usually with a circular to subcircular cross section.

The parispatium is a narrow opening or seam that forms between the inside of the connecting rings and the endosiphunclar deposits that grow forward and back from the region of the septal foremina.

About 45 species have been described from North America, including Greenland and the Canadian Arctic with Actinoceras margaretae, A. aequale, and A. gradatum the earliest known, coming from the lower Blackriveran Loweville fm of Ottawa.