Indigenous to the lower Amazon region, primarily the Parnaíba River of Brazil, Corydoras julii is found in flooded forest regions as well as creeks and small tributaries. It is often confused with Corudoras trilineatus, a species that is found in the upper Amazon.

Like similar Cory species, the body is silvery gray. A dark zigzag stripe runs along the lateral line from the gills to the tail. Above and below this dark line is a section that is not spotted, beyond which are many small dark spots of varying sizes. On the body, some of these spots connect to form short lines, but on the head, the spots are distinctly separate, a feature that differentiates this species from similar Corys.

The dorsal fin is transparent with a large black blotch on the upper fin that does not extend down into the body. The caudal fin has vertical rows of dark brown spots that give the appearance that it is striped. The anal, adipose, pectoral and ventral fins also have these spots, but they are much paler in color than on the caudal fin. Instead of scales, this species has overlapping hard plates, hence being called an armored catfish.