Continuous denitrification of nitrate-contaminated groundwater containing dissolved oxygen (DO), SO42−, and no buffer was carried out with three identical electrochemically activated biofilm reactors. The reactors consisted primarily of denitrifying biofilm attached to the surface of the cathode, an amorphous carbon employed as the anode, and a DC power supply. In the reactors, denitrification and neutralization caused by H2 and CO2 produced from the cathode and the anode, respectively, occurred simultaneously when an electric current was applied. A complete-mix reactor model coupled with a biofilm-electrode model was developed in conjunction with a limiting-current theory. When the biofilms were sufficiently acclimated and adapted to the electric current, the denitrification performance calculated using the model was in fairly good agreement with experimental results.