A highly efficient phosphorus-accumulating organism, Cupriavidus plantarum S7–1A, which can completely remove inorganic nitrogen, was isolated from municipal activated sludge. The S7–1A generated no gaseous nitrogen, was confirmed to produce no hemolysin, and showed high susceptibility to cephalosporin and floxacin antibiotics. Optimum conditions were obtained with sodium pyruvate as a carbon source, a C/N of 18, a pH of 7–9, a temperature of 30°C, a phosphorus concentration of 16.5 mg/L, and shaking at 250 rpm. The highest nitrate, nitrite, and ammonium removal efficiencies were respectively 99.94%, 99.98%, and 99.62%, and the corresponding removal rates were 4.87, 3.84, and 6.90 mg/L/h. Over 89% of total organic carbon was removed, and phosphorus was not detected at initial concentrations below 16.5 mg/L. Genome sequencing and PCR confirmed that S7–1A has nasA, nirB, nirD, ppk, ppk2, and ppx genes. In conclusion, S7–1A proved to be a safe and efficient assimilatory nitrate reduction and phosphorus-accumulating organism (ANR-PAO).