Hyloxalus nexipus males measure 20–24 mm (0.79–0.94 in) and females 19–23 mm (0.75–0.91 in) in snout–vent length. Dorsum is greenish black. Adult males have blacker throats than females. Juveniles have tiny white spots on the dorsum in the sacral region and on the legs. There is usually a dorsolateral as well as an oblique lateral stripe extending to eye.
Calling males have been observed during the daytime; one called from a stone at the edge of a river. The call consists of 20-29 notes, lasting 1–2 seconds. Females lay the eggs on land. Tadpoles are transported to streams by adults and develop in quiet pools in or adjacent to the watercourse. A newly metamorphosed juvenile frog measures about 12 mm (0.47 in).
