Wetlands have played a critical role in mitigating water and contaminants. Aquatic plants with high potential of removing nutrients and contaminants from water need to be evaluated for their use in a wetland. In this study, water lettuce (Pistia stratiotes) was evaluated for its potential to improve water quality. The plants were grown in a stormwater retention pond. Water samples were collected weekly from the control (without plant) and the remediation plot (with plant) for water quality analysis. Plant biomass was periodically harvested and the biomass yield was recorded. Plant samples were analyzed for the concentrations of N, P and metals, which were used for calculating total amounts of N, P, and metals removed by the plants. Phytoremediation reduced water turbidity by 60% and suspended solids by 10%. On average, total N and P in the water decreased by 0.63 and 0.32 mg/L (from 1.62 to 0.99 and 0.78 to 0.46 mg/L, respectively). Nitrate-N and PO4-P were averagely lowered by 0.13 (from 0.16 to 0.03) and 0.11 (from 0.44 to 0.33) mg/L, respectively. The concentrations of N, and P in the plant were averaged 18, and 3.3 g/kg, respectively. Mean concentrations of Fe, Mn, Zn, and Cu in the plant root were 5424, 714, 154, and 20 mg/kg, but were much lower in the shoot (333, 180, 68, and 3 mg/kg, respectively). Plant biomass yield (dry weight) was 46 Mg DW/ha•yr, which removed 721 kg N, 176 kg P, 587 g Cu, 110 kg Fe, 2.2 kg Zn, and 10 kg Mn per ha water area per year. These results indicate that water lettuce has a great potential for remedating stormwaters. It can effectively remove nutrients and improve water quality, and can also remove heavy metals such as Fe, Mn, Zn, and Cu by plant uptake and root adsorption.