The Polyrhachis (Myrmatopa) flavicornis species group in Thailand is reviewed. Four species are recognized, of which one species closely related to Polyrhachis constructor Smith, 1857 is described and illustrated as new to science based on the worker caste under the name Polyrhachis longipilosa sp. nov. The queens of P. flavicornis Smith, 1857 and P. constructor, and the males of P. flavicornis and P. varicolor Viehmeyer, 1916 are described. The new species is distributed in western Thailand in the area north of the Isthmus of Kra, while P. constructor is distributed in Malaysia (West Malaysia and Sabah), Indonesia, Singapore, southern and western Thailand. Polyrhachis piliventris Smith, 1857 and Polyrhachis elii Emery, 1900 are synonymized with P. constructor. A key to the Thai species of the Polyrhachis flavicornis species group is given. The type series of P. longipilosa was collected from the canopy of a dry evergreen forest.