<p>We report a simple voltammetric method using a boron-doped diamond electrode (BDDE) cathodically pretreated (CPT) to detect 2,4-dichlorophenoxyacetic acid (2,4-D) in river water samples from the Amazon region. The voltammetric response was evaluated as a function of anodic pretreatment (APT) and CPT for BDDE using the [Fe(CN)₆]^4‒/3‒ probe and 2,4-D. Electrochemical impedance spectroscopy (EIS) and potential scan rate results demonstrated that CPT enhances the voltammetric responses, particularly for 2,4-D, with a 220% increase in the electroactive area compared to the probe, attributed to interactions between the functional groups of 2,4-D and the electrode surface. Catalytic constant (<i>k</i>_cat) and faradaic capacitance (C_s) were determined for the first time for 2,4-D using CPT-BDDE. Furthermore, diffusion coefficient (<i>D</i>_app) and apparent heterogeneous electron transfer rate constant (<i>k</i>_app) were determined, and a possible mechanism for the oxidation of 2,4-D on the CPT-BDDE surface is proposed. CPT-BDDE exhibited good sensitivity, with a linear dynamic range of 0.1 to 260&#xa0;µM, and a limit of detection of 20&#xa0;nM. Furthermore, the sensor demonstrated good selectivity for detecting 2,4-D in the presence of interferents and exhibited good repeatability, reproducibility, and stability. The CPT-BDDE was successfully used to determine 2,4-D in river water samples, showing good recoveries and reliable results by the standard addition method. Finally, the proposed analytical method was evaluated using the Blue Applicability Grade Index (BAGI) and Click Analytical Chemistry Index (CACI) tools. The scores for BAGI and CACI were 80 and 73, respectively. These values reflect good robustness and feasibility of the proposed method.</p> Graphical Abstract <p></p>