Riocerida,[1] originally named as Rioceratida,[2] is an extinct order of Ordovician nautiloid cephalopods.

They were the earliest-diverging members of the major group Orthoceratoidea, with which they share dorsomyarian muscle scars (a few large muscle scars concentrated at the top of the body chamber).

[1][2] However, they also possess several plesiomorphic ("primitive") traits which are absent in other orthoceratoids: Cameral and endosiphuncular deposits are absent, and the siphuncle has a marginal position along the lower edge of the shell.

The conch (external shell) shape is slender, usually orthoconic (straight) or weakly cyrtoconic (slightly curved).

[2] The monophyly of Riocerida as a unique group is plausible but difficult to confirm, and they may instead be a paraphyletic grade or polyphyletic assemblage ancestral to all later orthoceratoids.