A new genus and a new species of Seirini (Entomobryidae) are described. The new genus, Tyrannoseira n. gen., is represented by three Brazilian Seira-like species with a peculiar type of sexual dimorphism: the first pair of legs of adult males bears several spine-like setae in distinct positions, the femur is enlarged and the tibiotarsus is slender. The combination of these morphological features produces clasping structures used during male-male interactions. The new genus is known only from Caatinga, a unique semi-arid biome exclusively present in Brazil. A new species, Tyrannoseira sex n. sp., is also described from São João do Cariri, Paraíba State. Seira raptora and S. bicolorcornuta, described from Caatinga, are also included in the newly described genus. An identification key for the three species of the new genus is also given. Sexual dimorphism and aspects of the systematics of Seira are discussed.