The current study evaluated nutrient removal efficiency, algae productivity, and economic feasibility related to cultivation of the filamentous algae, Hydrodictyon reticulatum, in a pilot plant-scale raceway pond with underwater light. Following the installation of an underwater light device, removal efficiencies of T-N and T-P increased by 11.0% and 12.3%, respectively. The productivity of algae also increased by 133.8% to 5.81 g/m(2)center dot d. Installation of the underwater light device increased the wastewater treatment capacity by over 3 times in the same area and enabled the removal of nutrients by over 3.5 times and removal of carbon dioxide by an additional 39 tons/yr. Estimates indicated that the cost of wastewater treatment and nutrient removal was reduced by more than 67%, while the production cost of H. reticulatum was reduced by 18% or more. Therefore, underwater light may solve issues related to the need for large sites in existing raceway ponds and enhance the economic efficiency by improving nutrient removal efficiency and productivity.