The development of a simple, fast, and sensitive method for Se(IV) determination is detailed in the present work. Experimental conditions were optimized using differential pulse and square wave cathodic stripping voltammetry. An ex-situ bismuth film was deposited on the surface of a glassy carbon electrode and used as the working electrode. An exhaustive study of potential interferents was performed. Cu(II) interference is one of the most significant challenges in Se(IV) determination. By integrating acetonitrile in the supporting electrolyte, Cu(I) was stabilized through complex formation, thereby preventing metallic copper deposition and copper selenide formation. Even with a 50-fold excess of Cu(II), no interference was observed in the quantification of Se(IV). The proposed method was validated using two certified reference materials (CRMs) and applied to the analysis of four water samples, three of which were spiked with Se(IV). Satisfactory results were obtained from the validation and water sample analysis.