We analyzed broadband waveform data recorded by a deep-sea borehole observatory (WP-1) and a long-term broadband ocean-bottom seismograph (NOT1) deployed in the west Philippine basin by the Ocean Hemisphere Project. We determined the depths of the 660-km discontinuity beneath the west Philippine basin using the receiver function method. The “660” depths determined from the WP-1 and NOT1 are consistent with each other, indicating that the estimated depths are reliable. The 660 depth determined using both WP-1 and NOT1 data was 669 ± 9 km, which is deeper by 9 km than the global averages, beneath the west Philippine basin. Interpreting the 660 depth in terms of temperature, the slightly deep 660 can be translated to mean lower temperatures by about 100 K at the 660, using the Clapeyron slope of the olivine to β-spinel and the post-spinel phase change. The cold temperature is qualitatively consistent with the tomographic image. When compared with previous regional studies of the 660 beneath the Philippine Sea, our results suggest the presence of significant topography on the mantle discontinuities beneath the Philippine Sea, which may be caused by a stagnant Pacific slab in the mantle transition zone. The present study demonstrates that data from deep-sea observations provide useful information for investigating deep Earth structure.