Thinobius andinus sp. nov. from Argentina is described and illustrated, detailed information on its ecology is provided and a key to species of Thinobius from south-western South America is given. This species was collected from high elevation stream areas of the Mendoza River watershed. It belongs to pollution-sensitive species, and it was mainly found in clear, low-salinity cold waters pertaining to high elevation streams with riparian vegetation. Lectotype is designated for Thinobius pulchripennis Bernhauer, 1923.