In spite of nearly 30 yr of investigation, the origin of the West Philippine Basin remains elusive. This basin occupies nearly half of the Philippine Sea plate, which remains one of the most poorly understood of the major lithospheric plates on Earth. Establishing the origin of the West Philippine Basin is fundamental to deciphering the evolution of the western Pacific Basin, including the India-Asia collision and the opening of numerous marginal basins in the region. In this paper we combine new on-land kinematic data from southwest Japan with existing marine magnetic data from the ancient Kula-Pacific Ridge to test an early tectonic model that suggests that the West Philippine Basin arose by trapping of a part of the Kula-Pacific Ridge during the middle Eocene. The results suggest that this mechanism is viable, but that the ridge-capture event may have occurred during the middle Eocene, ∼10 m.y. earlier than originally proposed.