The Saquarema archaeological site, on the Atlantic coast of the Rio de Janeiro State, is one of many shellmounds built on the Brazilian coast by hunter-gatherer populations during the Holocene. We used archaeological material from this site with the aim of evaluating the marine reservoir effect (MRE) in the region. Radiocarbon ages of 45 marine and 6 terrestrial samples from this shellmound provided data for assessing the MRE and the influences of freshwater and seasonal coastal marine upwelling in this specific locality. Samples of charcoal, fish otoliths, and mollusk shells were analyzed and the 14C dates were modeled in the OxCal platform to determine the marine reservoir correction. The result obtained is R = 265 ± 70 14C yr and the offset ΔR was found to be –140 ± 66 14C yr. To support the accuracy of this value for correcting conventional 14C marine ages, taxonomic analyses of the samples were performed.