A simple and fast electrochemical method was described and evaluated to determine the hazardous compound, 4-nitrophenol (4NP). In this work, concentration of 4NP was determined by differential wave voltammetry (DPV). A gold electrode (Au) was modified with 3,5-diamino-1,2,4-triazole (35DT). The modified electrode (35DT-Au) was characterized by using electrochemical impedance spectroscopy (EIS), fouirer transform infrared spektrofotometre (FTIR), cyclic voltammetry (CV) and DPV. The modified electrode showed more sensitivity towards 4NP compared to unmodified one. A wide linear concentration range from 0.24 to 130.6 mu M was obtained for 4NP with a detection limit of 0.09 mu M. In the reproducibility and repeatability studies, the relative standard deviation (RSD%) values of the method were obtained as 3.72 % and 2.56 %, respectively, which are acceptable values. This proposed method was successfully used for the analysis of 4NP in lake and tap water samples. Simplicity, sensitivity, selectivity and high efficiency of the proposed method can be used in routine analysis of trace amounts of 4NP in polluted waters.