Ag-phosphotungstic acid composite nanoparticles (Ag-PTA NPs) were synthesized by the reduction of silver nitrate with UV-irradiation in a PTA solution, followed by the assembly of the nanoparticles onto chitosan (CTS)-modified ITO-glass to be used for the electrochemical detection of 4-chlorophenol (4-CP). This modified electrode considerably increased the oxidation signals of 4-CP, which demonstrated its high electrocatalytic activity. The oxidation of 4-CP on the modified electrode is a reversible process controlled by surface absorption. The results of differential pulse voltammetry (DPV) showed that the peak current is linear with the 4-CP concentration in the range from 1 mu M to 0.4 mM, with a correlation coefficient of 0.999, a response sensitivity of 0.1247 mu A/mu M, and a detection limit of 0.34 mu M (S/N = 3). These results, combined with the electrode's good reproducibility, stability, and anti-interference performance, certainly make the Ag-PTA/CTS/ITO electrode a potential electrochemical sensor for the detection of chlorophenols.