A new species of the army ant genus Aenictus (Hymenoptera: Formicidae: Dorylinae) is described. Aenictus seletarius sp. nov., belonging to the Aenictus minutulus species group, was discovered from a single subterranean pitfall trap in Singapore. Like A. subterraneus and A. changmaianus in this species group, A. seletarius displays substantial variation in body size within the worker caste. An updated key to the Aenictus minutulus species group from Southeast Asia is presented.