Acleistoceras is a genus of the oncocerid, nautiloid family Acleistoceratidae that lived in the shallow seas that covered much of North America during the Devonian; living from 409—383.7 mya, existing for approximately 25.3 million years.

[1] Acleistoceras was named by Hyatt (1884) and assigned by Flower (1950) [2] to the newly established Acleistoceratidae which at that time was added to the Oncoceratida.

Sepkoski's listing of Acleistoceras in the Oncoceratida simply follows Flower (1950) and Teichert & Moore (1964).

[4] The maximum diameter is in the forward part of the phragmocone; the body chamber tapers slightly to a transverse, subtriangular aperture with a well-developed hyponomic sinus.

The siphuncle is subventral, composed of broadly expanded segments which gives it a nummuloidal or beaded appearance.