A cathodic electrochemical application of bismuth film modified glassy carbon electrode was investigated with the objective of determining thiamethoxam insecticide. The conditions of bismuth film deposition (electrode pretreatment procedure, composition of the plating solution, plating potential and film stability) were optimized using electrochemical and microscopic techniques. It was found that the bismuth film generation on glassy carbon substrate in a plating solution of 0.02 M Bi(NO₃)₃, 1 M HCl and 0.5 M KBr, at −0.25 V (vs. Ag/AgCl/3 M KCl) for 60 s and subsequent electrochemical conditioning yielded a modified electrode suitable for analytical purposes. Thiamethoxam was determined by differential pulse voltammetry in Britton–Robinson buffer of pH 8.0 in the concentration range of 1.26–45.0 μg/cm³. The reproducibility of the analytical signal was characterized by a relative standard deviation smaller than 1.5%, and the calculated values of detection and quantitation limits were 0.38 μg/cm³ and 1.26 μg/cm³, respectively. The applicability of the modified electrode was tested on real, agriculturally‐incurred potato and maize samples.