AbstractBlack carbon is ubiquitous in the marine environment. However, whether it accumulates in the deepest ocean region, the hadal zone, is unknown. Here we measure the concentration and carbon isotopes (δ¹³C and Δ¹⁴C) of black carbon and total organic carbon in sediments from six hadal trenches. Black carbon constituted 10% of trench total organic carbon, and its δ¹³C and Δ¹⁴C were more negative than those of total organic carbon, suggesting that the black carbon was predominantly derived from terrestrial C3 plants and fossil fuels. The contribution of fossil carbon to the black carbon pool was spatially heterogeneous, which could be related to differences in the distance to landmass, land cover and socioeconomic development. Globally, we estimate a black carbon burial rate of 1.0 ± 0.5 Tg yr⁻¹ in the hadal zone, which is seven-fold higher than the global ocean average per unit area. We propose that the hadal zone is an important, but overlooked, sink of black carbon in the ocean.