Ostracode assemblages of the Poty quarry, Pernambuco-Paraíba Basin, northeastern Brazil, record an abrupt faunal change near the Cretaceous-Paleogene (K-Pg) boundary. Paleoecological and stable isotope analyses of ostracode carapaces were used to interpret the paleoenvironmental changes that took place from the Late Maastrichtian to the Early Danian. Ostracode distribution demonstrates substantial changes at and around the boundary with the last occurrences of most species and appearance of new ones. Stable isotope analyses carried out for four ostracode species distributed along the entire succession exhibit oscillations in the isotopic composition before the K-Pg boundary, thus suggesting that environmental changes may have begun slightly prior the boundary event itself. These changes may have triggered the extinctions that culminated at the K-Pg boundary, marked by positive δ18O and negative δ13C excursions. The K-Pg boundary is marked by cooling of bottom seawaters and a decrease in productivity, as well as the beginning of an extensive marine regression.