A soldier-based key to the South American species of Heterotermes is presented. Six species are recognized: H. assu sp. n., H. convexinotatus, H. crinitus, H. longiceps, H. sulcatus and H. tenuis. H. assu sp. n., is described from the Brazilian Atlantic forest, including the imago, soldier and worker castes. H. assu is also recorded from urban areas as a pest. The imago of H. longiceps is described and illustrated for the first time. The soldiers of all species are illustrated and their known distribution mapped, with several new records.