Coelogaster is an extinct genus of marine ray-finned fish that lived during the early Eocene.

It contains a single species, C. leptostea, known from the famous Monte Bolca site of Italy.

[1][2] It is classified in the Anotophysi, and is generally considered a chanid of uncertain affinities, making it related to modern milkfish.

[3][4][5][6] It was initially named without formal description by Louis Agassiz in 1835 as Clupea leptostea, alongside another fish known as Coelogaster analis.

In 1905, Eastman officially described C. analis based on Agassiz's original name, and also described Clupea leptostea under the new genus Chanoides.