In the current study, we developed a nanocatalyst-based electrochemical immunoassay using magnetic beads (MBs) and gold nanocatalysts (AuNs). The MBs conjugated with IgG allow easy separation of target proteins and rapid immunosensing reaction, and the AuNs conjugated with IgG amplifies electroactive species via catalytic reaction of AuNs. An antimouse IgG-MB conjugate and an antimouse IgG-AuN conjugate sandwich a target mouse IgG with low nonspecific binding. Thus formed immunosensing complex is strongly attracted to an indium tin oxide (ITO) electrode modified with partially ferrocenyl-tethered dendrimers (Fc-Ds) by using an external magnet. The AuN of the immunosensing complex produces p-aminophenol from p-nitrophenol by catalytic reduction in the presence of NaBH(4), and the generated p-aminophenol is electrooxidized at the Fc-D-modified ITO electrode. The oxidized product, p-quinone imine, is reduced back to p-aminophenol by NaBH(4) and then re-electrooxidized at the electrode. This redox cycling greatly amplifies the electrochemical signal. Moreover, the Fc-D-modified ITO electrode exhibits a low background current. Accordingly, the high signal-to-background ratio allows an extremely low detection limit of 1 fg/mL (7 aM) in cyclic voltammetric experiments and, importantly, 100 ag/mL (0.7 aM) in differential pulse voltammetric experiments.