This paper presents results of a seismic tomography experiment carried out on the accretionary margin off southwest Taiwan. In the experiment, a seismic air gun survey was recorded on an array of 30 ocean bottom seismometers (OBS) deployed in the study area. The locations of the OBSs were determined to high accuracy by an inversion based on the shot traveltimes. A three-dimensional tomographic inversion was then carried out to determine the velocity structure for the survey area. The inversion indicates a relatively high P wave velocity (Vp) beneath topographic ridges which represent a series of thrust-cored anticlines develop in the accretionary wedge. The bottom-simulating reflectors (BSR) closely follow the seafloor and lies at 325 ± 25 m within the well-constrained region. Mean velocities range from ~1.55 km/s at the seabed to ~1.95 km/s at the BSR. We model Vp using an equation based on a modification of Wood’s equation to estimate the gas hydrate saturation. The hydrate saturation varies from 5% at the top ~200 m below the seafloor to 25% of pore space close to the BSR in the survey area.