About 15 Tg of plastic are estimated to enter the ocean yearly, with this figure growing exponentially every year. Assessments of floating marine plastic amount to 0.2% of the expected plastic stock, implying that 99.8% is stored in a sink yet to be identified. Marine sediments are believed to be the ultimate sink for oceanic plastic, however, there is currently no global estimate of the plastic load stored there. Here, we synthesize available estimates of micro- and mesoplastic stocks in marine sediments and, by integrating stocks across different habitats, we conservatively estimate a load of 170 Tg (25–900 Tg) of nonfibrous plastic globally accumulated in marine sediments from 1950 to 2010, most of which at intermediate depths (200–2000 m). This estimate, despite the uncertainty, is two to three orders of magnitude higher than the floating plastic stock and confirms marine sediments as a major plastic sink.