The fate of black carbon (BC), a stable form of thermally altered organic carbon produced during biomass and fuel combustion, remains an area of uncertainty in the global carbon cycle. The transfer of photosynthetically derived BC into extremely long‐term oceanic storage is of particular significance and rivers are the key linkage between terrestrial sources and oceanic stores. Significant fluvial fluxes of dissolved BC to oceans result from the slow release of BC from degrading charcoal stocks; however, these fluvial fluxes may also include undetermined contributions of aerosol BC, produced by biomass and fossil fuel combustion, which are deposited in river catchments following atmospheric transport. By investigation of the Paraíba do Sul River catchment in Southeast Brazil we show that aerosol deposits can be substantial contributors to fluvial fluxes of BC. We derived spatial distributions of BC stocks within the catchment associated with soil charcoal and with aerosol from both open biomass burning and fuel combustion. We then modeled the fluvial concentrations of dissolved BC (DBC) in scenarios with varying rates of export from each stock. We analyzed the ability of each scenario to reproduce the variability in DBC concentrations measured in four data sets of river water samples collected between 2010 and 2014 and found that the best performing scenarios included a 5–18% (135–486 Mg DBC year−1) aerosol contribution. Our results suggest that aerosol deposits of BC in river catchments have a shorter residence time in catchments than charcoal BC and, therefore, contribute disproportionately (with respect to stock magnitude) toward fluvial fluxes of BC.