The Pacific leaping blenny (Alticus arnoldorum) is a marine fish that has made a highly successful transition to land. We report an extensive field study on the behavior of this remarkable fish and how it has coped with life on land. The fish occurs in great abundance above the waterline along the rocky coastlines of Micronesia. We found them to be terrestrial in all aspects of their adult daily life, but heavily constrained by large fluctuations in both tide and temperature with almost all activity limited to a brief period at mid-tide. Despite this, the fish were highly social and data were consistent with males defending exclusive territories on land. A variety of metrics – the use of visual displays, the allometry of ornaments, and sexual dimorphism – further imply sexual selection on both sexes was strong. Despite being restricted to an extremely narrow habitable zone in which conditions change constantly, the Pacific leaping blenny is remarkably adapted to life on land and rarely returned to water. The genus is unique among the living fishes in its degree of terrestriality and serves as a useful model of the constraints and adaptations that accompany major ecological transitions.