[1][2] The frog's specific name, japi, comes from a Tupi word meaning "springs", referencing the breeding environment of this species.

The digits bear lateral fringes, more extensive on the toes than on the fingers, and more pronounced in males than females, as well as weakly developed scutes.

Although Hylodes japi is mostly silver in color, its body features dark spots and blotches, light, lateral stripes, and brown to chestnut bars.

[2] Overall, this species can be distinguished from other members of its genus by a continuous oblique lateral fold as well as a lack of tubercles near it, a light-colored stripe, smoother dorsal surfaces, advertisement calls, and the ventral coloration.

[2] Hylodes japi occurs at 850–1050 meters above sea level, near swift rivulets, in semideciduous, mesophytic forest.

If the female accepts the male's offer, the latter digs up a tunnel and a chamber in the sand of the riverbed, big enough for the two frogs, in which the intercourse occurs.

Their breeding chambers, inside of which they remain during the early stages of development, are abandoned after the formation of limb buds.