A new minute salamander of the genus Thorius is described from the western highlands of the Mexican state of Guerrero. Previously, three congeners were known from the central portion of the state. The fourth species we herein describe occurs in the western portion of the Sierra Madre del Sur of Guerrero and is allopatric from nearest congeners by about 80 km. The new species is distinguished from Guerreran congeners by a combination of body size, external morphology, and dentition.