Biological treatment of nitrate in reverse osmosis (RO) concentrate produced from municipal wastewater effluent is challenging, in part because of the low carbon-to-nitrogen ratio. Open-water unit process wetlands may provide a cost-effective means of removing nitrate because autotrophic production of labile organic carbon supports denitrification in the wetland biomat. To determine the potential for employing open-water unit process wetlands for removing dissolved nitrogen species from RO concentrate, a pilot-scale open-water wetland treatment system was established and studied over a two-year period at a water reuse facility in San Jose, California. The system was operated with a 3-day hydraulic residence time, resulting in removal of up to 30% of the nitrate present in the RO concentrate during the first summer of operation and removal of up to 47% of the nitrate during the second summer. The biomat comprised a diverse algal and heterotrophic bacterial assemblage containing several clades that are putatively capable of denitrification, as well as greater abundances of denitrifying functional genes (nirK, narG) in the second year, coincident with higher nitrate removal. In batch reactors, the addition of woodchips increased nitrate removal rates from RO concentrate by approximately a factor of five or more, with rates dependent on the dose of woodchips applied. These results indicate that woodchip amendments could reduce the land area needed for nitrate treatment. This study provides evidence that open-water wetlands can remove nitrate from RO concentrate at the pilot scale, and identifies opportunities to enhance treatment efficiency with low-cost carbon amendments.