Radiocarbon dating of closely associated marine mollusk shells and terrestrial material (mammal bones or charred wood) collected from archaeological contexts in northern Atlantic Iberian coastal areas is used to quantify the marine 14C reservoir effect (ΔR) for the coastal waters off the Cantabrian coast of northern Iberia. For the first time, ΔR values were reliably determined for these coastal waters and, also for the first time, a ΔR was calculated for the Late Pleistocene in Atlantic Iberia. Pairs of coeval samples of different carbon reservoirs selected from Upper Paleolithic (Late Pleistocene) and Mesolithic (Early Holocene) contexts yielded ΔR weighted mean values of -117±70 14C yr and -105±21 14C yr, respectively. These values show oceanographic conditions characterized by a reduced offset between atmospheric and surface water 14C contents, suggesting a nonexistent or very weak upwelling and some stratification of the water column. Similar oceanographic conditions have been recorded in other areas of Atlantic Iberia during the Holocene, such as off Andalusian and northwestern Galician coasts. Results not only provide useful information on environmental conditions but also a framework to obtain more precise and reliable absolute chronologies for the Late Pleistocene and Early Holocene in northern Iberia.