Knowledge of the abundance, source, and fate of marine debris in the deep sea is largely constrained thus far. Here, we report the existence of large deep-sea debris dumps that have not been reported before on the seafloor worldwide. Marine debris remarkably accumulated at similar to 1700-1800 m in the tributary submarine canyons of the Xisha Trough, northern South China Sea (SCS). Although marine debris in Xisha Trough is patchy, the debris abundance was as high as 36,818 and 51,929 items/km(2) at locations SY78 and SY82, respectively, which is one order of magnitude higher than that in other submarine canyons. We propose that most of the debris came from fishery and navigation activities, as indicated by the categories of debris collected from the seafloor dumps. Seasonal surface ocean currents of the SCS and geomorphology of submarine canyons possibly influence the movement of the debris from coasts to the deep seafloor.