Organic carbon (OC), total nitrogen (TN), and carbon isotope ratio (δ13C) were measured in sediment cores to constrain the relative contribution of marine and terrestrial sources of organic matter (OM) in the continental shelf of the Gulf of Cádiz. Five sampling stations were selected in two different areas: the Bay of Cádiz and its outer region (BOX BC02, BOX BC04, BOX BC05), and the Guadalquivir prodelta (BOX GL03, BOX GL04), which is characterized by a high fluvial supply and a moderate hydrodynamic regime. C/N and δ13C data show that sedimentary OM is a mixture of marine and terrigenous OM with larger contributions from marine sources in the Bay of Cádiz, and terrestrial sources in the Guadalquivir prodelta.