Seismic imaging of subduction zones can provide constraints on mineral reactions in the slab and surrounding regions. We use P-to-S converted phases from teleseisms recorded at broadband stations in the Mariana Islands to image the forearc and arc regions of the Mariana convergent margin. The subducting oceanic crust is observed between 75 and 110 km depth as a thin low velocity zone overlying the subducting Moho, demonstrating that the basalt to eclogite phase transition must occur at a greater depth. A low velocity zone (LVZ), approximately 10−25 km thick, whose upper boundary is imaged at about 40−55 km depth, is detected in the forearc region of the mantle wedge along the entire margin. The anomaly is located too shallow to represent subducted oceanic crust. We interpret the LVZ as a serpentinized region in the forearc mantle, resulting from hydration by slab-expelled water. The inferred S-wave velocity in the LVZ of as low as ∼3.6 km/s represents a level of serpentinization of 30−50%, corresponding to a chemically bound water content of about 4−6 wt%.