Mecyclothorax palikea, sp.n. is described from the vicinity of Palikea, southern Waianae Range, Oahu, HI, USA and is assigned to Britton's Mecyclothorax flavomarginatus species group. Cladistic analysis, based on 20 morphological characters and including several outgroup taxa, places Mecyclothorax impunctatus Liebherr of Molokai as adelphotaxon to the other eight species of the group, with subsequent speciation events successively isolating M. sharpi Britton of West Maui versus a clade of seven Oahu species. Phylogenetic relationships among the Oahu clade species posit three historical speciation events vicariating ancestors on the western Waianae and eastern Koolau Ranges. Mecyclothorax palikea is placed as adelphotaxon to M. carteri (Perkins), a species allopatrically distributed to the north in the Waianae, corroborating existence of southern and northern areas of endemism within the Waianae Range. Relative ages of the respective volcanoes housing M. flavomarginatus group species — Waianae (3.7 million years ago), Koolau (2.6 million years ago), Eastern Molokai (1.8 million years ago), West Maui (1.3 million years ago)—imply that ancestral occupation of Oahu by this group occurred subsequent to completion of the shield building phases of Oahu's two volcanoes, Waianae and Koolau. Diversification within the group on Oahu was associated with vicariance events that occurred within a terrestrial environment. Whereas all four species of the M. flavomarginatus group occupying Waianae Range habitats have been observed recently in nature, collection of M. flavomarginatus in 1906 represents the most recent record for any M. flavomarginatus group species in the Koolau Range, indicating the importance of conserving appropriate Waianae Range habitats in order to preserve representative biodiversity in this species group.