Burial of organic carbon (OC) in marine sediments moderates atmospheric CO2 levels on geological time scales, but uncertainties remain about how much OC is buried and about the efficiency of OC burial, particularly in heterogeneous seafloor environments such as ocean margins. Here we describe OC burial in Nazaré submarine canyon and the adjacent continental slope off Portugal, an area within which sedimentation rates vary by three orders of magnitude. Using a nested series of observations at different scales, ranging from regional bathymetry to sediment cores, we estimate the annual sediment and OC deposited in the canyon at 620,000 t and 12,500 t, respectively. Nazaré Canyon is thus a significant sink of both sediment and OC. Canyon sediments typically contain ∼2% OC, both in surface sediments and at depth, and there is a limited correlation between sedimentation rate and OC content. The likely explanation is that the OC has already survived a lengthy period of degradation prior to deposition in the canyon, such that additional exposure to oxygenated water has minimal effect. Burial efficiency is difficult to calculate because of extensive resuspension and reworking of OC in the upper canyon, but probably exceeds 30% in areas of high sedimentation. These areas are shown to be 30 times more effective in burying OC than adjacent areas of the continental slope, indicating that Nazaré Canyon is a hitherto overlooked sink of OC on a continental margin where OC burial is otherwise low.