Eusemius is an extinct genus of prehistoric marine ray-finned fish, previously described as a macrosemiid, that lived during the Late Jurassic. It contains a single species, E. beatae, from the Tithonian-aged Eichstätt Formation of Germany. It is known from a single very small specimen, likely a juvenile, that is likely now lost. It closely resembles the co-occurring Ophiopsis and Ophiopsiella (which are now classified in the Ionoscopiformes), only differing in the non-bifurcated dorsal fin rays.