Eocoelopoma gigas was the giant of its genus and a sizeable predatory fish in the London Clay sea. The head alone was much larger than in other Eocoelopoma species, and the jaws carried slender sharp teeth suited to seizing prey. Its size and dentition imply an active predator capable of taking comparatively large fishes or other nekton, although no direct gut-content evidence is known.

The skull was relatively smooth in several areas, with a distinctive projecting vomer at the front of the palate. Three-dimensional preservation allows these differences to be recognised despite the absence of a complete body. The species shows that considerable body-size disparity had already developed among early Eocene mackerel-like fishes.