Casierius is an extinct genus of marine ray-finned fish that lived during the Albian stage of the Early Cretaceous epoch. It was a relative of the modern bonefish in the extinct family Phyllodontidae, although some authorities consider it either a true albulid (making it even more closely related to bonefish) or a very early eel. It contains a single species, C. heckeli, known from the Glen Rose Formation near Hood County, Texas.