The Neogene-Holocene tectonics of southwest Japan have been controlled by the interaction of the Eurasian, Philippine Sea, and Pacific plates, which now meet at the central Japan trench-trench-trench (TTT) triple junction. Previous plate models have depicted the Shikoku back-arc basin as opening at the eastern margin of the Philippine Sea plate, parallel to and against the Japan margin. These models consider the TTT triple junction as predating the Shikoku Basin, but they predict neither the evolution of this junction nor the nature of the southwest Japan plate boundary during basin opening. Our recent work on the Oligocene-Miocene strata at Cape Muroto, Shikoku Island, has allowed us to locate the approximate position of the Shikoku Basin spreading ridge at 15 Ma. Combined with a compilation of the geology of the early Miocene southwest Japan margin, this determination leads us to propose that the Shikoku Basin was separated from the Japanese margin by an extension of the Pacific plate during basin opening. The basin subsequently collided with southwest Japan at ca. 15 Ma; this model implies that the central Japan triple junction was generated during this collision.