Denitrification, the anaerobic microbial conversion of nitrate (NO3−), a common water pollutant, to nitrogen (N) gases, is often high in the soil of natural wetlands. In areas where natural wetlands have been degraded or destroyed, constructed and restored wetlands have been used to restore ecosystem services like denitrification. Thus, denitrification in restored and constructed wetlands could play an important role in treating anthropogenic N sources such as combined sewer overflow discharges which can be high in NO3−. In this study, we measured denitrification potential using an anaerobic slurry assay and made a suite of ancillary measurements (soil moisture content, microbial biomass carbon [C] and N content, potential net N mineralization and nitrification, soil inorganic N pools, and soil respiration) in four constructed salt marsh wetlands, and a series of wetland habitat basins in Newtown Creek, NY, an urban superfund site. Samples were also taken from natural salt marshes located at Paerdegat Basin, Jamaica Bay, NY. Our results show that constructed Spartina alterniflora marshes in ultra-urban Newtown Creek support denitrification potential equivalent to rates of natural marshes in Jamaica Bay and reference marshes in other urban estuaries. There were significant positive correlations between microbial biomass C and N content and organic matter content and denitrification potential. Results suggest that constructed wetlands can support wetland vegetation, soils, and microbial life and contribute to N removal under ultra-urban conditions.