Masillosteus ("bony one from Messel") is an extinct genus of gar that inhabited western North America and Europe during the Eocene.

It is known from two species, each from a famous freshwater lagerstätte: M. kelleri from the Messel pit in the Messel Formation of Germany, and M. janeae from Fossil Butte in the Green River Formation of Wyoming.

[1][2] They are known from only a few specimens from both localities, and may have not been permanent inhabitants of the fossil lakes where they were preserved.

[3][4] Masillosteus was an atypical gar with a short, broad snout and molariform teeth likely adapted to crushing crustaceans and other hard-shelled invertebrate prey.

[1] It shares the short, broad snout with the extinct gar Cuneatus, which inhabited western North America during the same time period; both genera are classified in the tribe Cuneatini.