The aim of this study is to investigate the stable isotope composition of precipitation and river water from the northeastern part of Romania. For this study, we collected monthly samples (for variable periods of time) of precipitation from six stations, and river water from three stations, between March 2012 and December 2017. The precipitation in the area is sourced mainly from the Atlantic Ocean, and secondarily from the Black Sea, local recycling being important especially in summer. We found that the seasonal δ18O in precipitation is in agreement with the seasonal temperature variability, as shown by the significant correlation coefficient between the two variables (r = 0.77), which indicates that the temperature has an important role in the δ18O variability in precipitation water in this region. The local meteoric water line in the northeastern part of Romania is defined by the equation δ2H = 7.80 × δ18O + 7.47, (r2 = 0.99, n = 121). The results presented in this study emphasize that the δ18O (and δ2H) and d-excess variability are strongly influenced by temperature, precipitation and the prevailing large-scale atmospheric circulation.