The sodium salt of 7-ethylthio-3-nitro-1,2,4-triazolo[5,1-c]-1,2,4-triazin-4-one dihydrate (1) is one of promising antiviral compounds that have shown a pronounced biological activity against influenza virus subtype H5N1, West Nile, and SARS (severe acute respiratory syndrome). Using cyclic voltammetry, it was shown that the electrochemical activity of compound 1 in a Britton–Robinson buffer solution (BRB) of pH 2.0 on a glassy carbon electrode is due to the electrochemical reduction of the nitro group conjugated with the heterocyclic system. A method was developed for the determination of compound 1 by direct cathodic square-wave voltammetry. The linearity region of the calibration curve in a BRB solution of pH 7.0 is 10–300 mg/L (R2 = 0.999), the limit of detection is 1.5 mg/L, and the limit of quantification is 4.5 mg/L. The accuracy of the developed procedure is close to 100%, the relative standard deviation is 3.6%.