A new species of Tyrannomyrmex (Hymenoptera, Formicidae), T. alii sp. nov., is described from the Western Ghats of Kerala State in southern India. This new species can be distinguished from other Tyrannomyrmex species using morphological characters, from T.dux by the petiolar shape, and from T. legatus and T.rex by surface sculpture and pilosity. The gyne of the genus is described for the first time. We provide detailed morphological descriptions of the gyne and worker castes, images from scanning electron microscopy, a key to all four known species of Tyrannomyrmex, and a note on the ecology of this rare genus. Tyrannomyrmex is a rare myrmicine ant genus distributed in the Indomalayan bioregion, with its range extending from southern India and Sri Lanka to Southeast Asia (General & Alpert 2012). The genus was erected for the new species, Tyrannomyrmex rex Fernández, discovered from Malaysia, Negri Sembilan; Pasoh Forest Reserve (Fernández 2003). Two other species have been described, T. dux Borowiec from the Ponmudi Hills in the Western Ghats of Kerala State in Southern India (Borowiec 2007), and T. legatus Alpert from the Sinharaja Forest Reserve in Sri Lanka (Alpert 2013). Later T. rex was also discovered from Singapore (Jacquemin et al., 2015). All three species were described from single specimens collected from leaf-litter samples in evergreen forests. Detailed information on the ecology or habitat of Tyrannomyrmex is still lacking. Alpert (2013) placed the genus in Solenopsidini Forel 1893, near the genus Monomorium, following morphological characters as per Bolton (2003). This hypothesis was partially supported by the phylogenetic analysis of Ward et al. (2015), which recovered Tyrannomyrmex within a clade containing Epelysidris and Erromyrma, both containing species formerly placed in Monomorium, and all three nested within the newly defined Solenopsidini. The genus was known from the worker caste and a possible male from the Philippines (General & Alpert 2012; Alpert 2013). Here we describe a new species, T. alii sp. nov.. We also describe the gyne of the genus for the first time and provide much needed ecological notes. A key to all known species of Tyrannomyrmex is provided based on worker morphology.