Rioceras is an extinct genus of orthocerid cephalopods of the family Baltoceratidae that lived in what would be North America during the Early Ordovician from 480—472 mya, existing for approximately 8 million years.

It was assigned to Baltoceratidae by Flower (1964) and Hook and Flower (1977),[2][3] but reassigned to the Ellesmeroceratidae by Kröger et al. (2007)[4] Whether Rioceras is assigned to the Baltoceratidae and therefore to the Orthocerida or to the Ellesmeroceratidae depends on the definition of each.

Flower[2] placed Rioceras in the Baltoceratidae when he proposed the genus since it lacks diaphragms in the siphuncle which for him was a defining character of the family.

Kröger[4] removed Rioceras to the Ellesmeroceratidae on the basis of its inwardly thicked connecting rings which produce concave segments, a defining character of that family as emended.

Flower did point out in his original diagnosis[2] that Rioceras has moderately thick connecting rings "of ellesmeroceratoid aspect" The shell is usually long, and may be straight ("orthoconic") or gently curved.