Thomas’ ichthyomyine is a stream-associated rodent. Compared with a close relative, it tends to have broader hind feet and a tail longer than its head and body.
Compared with the related Rheomys raptor, its hind feet are longer and broader, and its tail tends to exceed head-and-body length. Its dorsal fur is thicker and more coarsely grizzled, with more silver-tipped hairs on the belly.
Species-specific natural-history records are limited. Ichthyomyines as a group typically forage in clear-running mountain streams and rivers, where they prey mainly on aquatic arthropods.
An extinction-risk assessment has not been added yet.