We use P-to-S converted phases from teleseimic data recorded at island and ocean bottom stations in Mariana to investigate the upper mantle structure in the region. We find evidence for double seismic discontinuities at the base of the transition zone near the Mariana slab. A shallower discontinuity is imaged at depths of ∼650–715 km and a deeper interface lies at ∼740–770 km depth. The large lateral extent at near constant depths for both features is consistent with horizontal interfaces rather than small-scale scatterers. The amplitude ratios of the seismic signals suggest that the shear velocity contrast across the two interfaces is comparable. These characteristics support the notion that the discontinuities are the results of phase transformations in olivine (ringwoodite to post-spinel) and non-olivine component (ilminite to perovskite), respectively, for the pyrolite model of mantle composition.