The taxonomy of an invasive pest species, the Argentine ant, is reviewed. Linepithema humile (Mayr) 1868 is confirmed as the valid name for the Argentine ant. Morphological variation and species boundaries of L. humile are examined, with emphasis on populations from the ant’s native range in southern South America. Diagnoses and illustrations are provided for male, queen, and worker castes. Collection records of L. humile in South America support the idea of a native distribution closely associated with major waterways in lowland areas of the Paraná River drainage, with recent introductions into parts of Argentina, Brazil, Chile, Colombia, Ecuador, and Peru.