The Okinawa Trough is a backarc basin located behind the Ryukyu arc-trench system. The southernmost part of the Okinawa Trough (SPOT) displays different tectonic features from the rest of the Okinawa Trough. The SPOT area includes abundant seamounts with active hydrothermal venting and high heat-flow values. To understand better the rifting and magmatism context of the SPOT area, we examine the lithospheric structure, buoyancy and coupling across the southernmost Ryukyu subduction zone. The results show that beneath the SPOT area the continental crust and mantle lithosphere thickness of ~ 25-30 and 120 km, respectively, are thick with little continental thinning. The analysis of mantle lithosphere buoyancy across the southernmost Ryukyu subduction zone shows strong plate coupling between the overriding and subducting plates. However, the two plates are actually decoupled as indicated by present-day interface earthquakes. This situation indicates that the southernmost Ryukyu subduction zone displays a transitory case of a changing plate coupling, from a strong to a weak plate coupling. Such a coupling/decoupling transition is probably associated with the collision of the Luzon are with the Asian continental margin. Additionally, the curve of the mantle lithosphere buoyancy across the southernmost Ryukyu subduction zone indicates that the submarine magmatism in the SPOT area is located within the volcanic are area, suggesting early are magmatism in the SPOT area.