It has an elongated, vertically flattened body with a long whip-like tail filament. It is green dorsally, grading to silvery white ventrally, with two blue stripes or rows of blue spots along the back. The dorsal and anal fins are transparent at the base, while the caudal filament is white. At night, the body becomes broadly banded.

It looks like Fistularia tabacaria (Atlantic cornetfish). This species can be distinguished by the distinct grooves or bony plates (scutes) along the back. Fistularia commersonii has a smoother back, lacking these prominent dorsal grooves, and typically shows two thin blue longitudinal stripes or rows of blue spots on a greenish-grey to brown body.