Thomasomys caudivarius is a mouse of the western Andes from Ecuador to northern Peru. It has soft brown back fur and a long tail with a white tip. It is described as nocturnal and terrestrial, feeding on seeds and insects.
It is medium-sized, with soft, dense, fairly long brown fur on the back and cream-colored fur underneath. The tail is longer than the head and body and ends in a white tip.
It is described as nocturnal and terrestrial and feeds on seeds and insects. Recorded settings include montane forest and páramo at about 2,750–3,350 m.
An extinction-risk assessment has not been added yet.