This work investigates the 223Ra, 224Ra, 226Ra and 228Ra isotope distribution in river, estuarine waters and sediments of the Paranagu?? Estuarine Complex (PEC). The stratification of the Ra isotopes along water columns indicate differing natural sources. In sediments, the radium isotope activities was inversely proportional to the particle size. The highest concentrations of 223Ra, 224Ra, 226Ra and 228Ra in the water column were found in the bottom more saline waters and towards the inner of the estuary. These relatively high concentrations towards the bottomof the estuarymay be attributed to the influence of tidally driven groundwater source and desorption from particles at the maximum turbidity zone. The apparent river water ages from the radium isotope ratios, 223Ra/224Ra and 223Ra/228Ra, indicate that the principal rivers that flow into the estuary have residence times from between 6 and 11 days.