In this study, published and unpublished data on bulk organic matter from surficial sediments along the Southwestern Atlantic Margin (SAM) were gathered to identify sources and understand carbon transport over this large and complex area under the regime of strongly coupled climatic-hydrological-oceanographic interactions. To achieve this goal, total organic carbon (TOC), total nitrogen (TN), stable carbon (δ¹³C), and nitrogen (δ¹⁵N) isotopes, as well as calcium carbonate data from 242 sedimentary samples over the SAM were analyzed. Cluster analysis defined three geochemically distinct groups, closely related to sedimentary and hydrodynamic controls. Our results show that the latitude of 28°S marks a transition region in terms of the sedimentary bulk organic matter. To the south, organic material is strongly influenced by the continental contribution from the La Plata River and by the action of the Brazilian Coastal Current, whereas to the north, the contribution of La Plata River gradually decreases. Oceanographic fronts, currents, and river supply are the main mechanisms that drive organic matter distribution over the SAM.