We investigated the seismic velocity discontinuities in the uppermost mantle of Kyushu, a subduction zone in Japan, using receiver function analyses developed especially for discontinuities with high dipping angles. We elucidated the regional variation of discontinuities, as being the continental Moho, oceanic Moho, and upper boundary of the Philippine Sea slab. From the geometry of these discontinuities and contrast in S wave velocities, we discovered that the oceanic crust of the Philippine Sea slab has a low velocity and is possibly hydrated down to 70 km beneath south Kyushu, 80–90 km beneath central Kyushu, and less than 50 km beneath north Kyushu. The fore-arc mantle wedge beneath central Kyushu has a low-velocity zone and possibly contains hydrated materials and free fluid but such a low-velocity zone does not exist in the fore-arc mantle wedge beneath north Kyushu and south Kyushu. Beneath south Kyushu, water dehydrated from the slab could move to the back-arc side and cause arc volcanism. Beneath central Kyushu, water dehydrated from the slab could move to the fore-arc side and cause a gap in volcanism and hydration of fore-arc mantle materials. Beneath north Kyushu, the oceanic crust does not appear to convey water abundantly in the mantle wedge. The low-velocity hydrated fore-arc mantle extends landward beneath the northern part of central Kyushu and could produce volcanic rocks partially contaminated by slab-derived fluid.