Since the 1940s, use of road salt as a deicing agent has increased substantially in regions of the US with cold winters. Despite its ubiquitous application and known negative consequences for aquatic and human health, little research has documented the effects of road salt on the water quality of either streams or groundwater in regions, such as New Hampshire (NH), with harsh northern climates. We measured stream Na and Cl⁻ concentrations in 44 basins spanning a gradient of urbanization in southeastern and central NH. Among all sampled basins, stream Na and Cl⁻ concentrations were highly correlated with basin % road pavement (r²  =  0.75 for Na , and 0.78 for Cl⁻). In southeastern NH, concentrations also were correlated strongly with % impervious surface (r²  =  0.86 for Na , and 0.92 for Cl⁻). Groundwater salt concentrations in 143 private wells were significantly correlated with % impervious surface within a 500-m radius of each well, but the proportion of explained variance was small (r²  =  0.07 for Na , and 0.10 for Cl⁻). Concentrations of salt in streams and groundwater were surprisingly high. Mean concentrations of Na ranged from <1 to 298 mg/L and of Cl⁻ ranged from <1 to 573 mg/L. Mean Cl⁻ concentration in 1 small stream exceeded the US Environmental Protection Agency (EPA) chronic toxicity standard of 230 mg/L, and 9% of groundwater samples exceeded the secondary EPA maximum contamination levels for drinking water (250 mg/L of either Na or Cl⁻). In our long-term study basin, the Lamprey River, specific conductance increased over the period from 1978 to 2008, a result that indicated a corresponding increase in Na and Cl⁻ concentrations. Both Na and Cl⁻ concentrations in the Lamprey River were negatively correlated with flow, but the slope of the relationship decreased after a significant flood in 2006. Our data suggest that road-salting practices are contributing to the salinization of stream water and groundwater in NH, and that hydrologic variability, which is predicted to increase with climate change, could strongly affect the degree of salinization observed in surface waters.