A Ni(II) complex, [Ni^II(Me₄-NO₂Bzo[15]tetraeneN₄)], was used for electrocatalytic reduction of CO₂ in acetonitrile solvent. Then, the reduced form of CO₂ (CO₂⁻) was used for selective carboxylation of phenylacetylene to produce cinnamic acid at room temperature. The potential of the process is significantly less negative in comparison with those reported earlier. Using sacrificial magnesium electrode as anode, controlled potential coulometry was carried out in an undivided glass cell. The spectral characterizations of FTIR, ¹H NMR, and ¹³C NMR demonstrated that cinnamic acid was the main product of the electrolysis. With respect to other catalysts, which have been previously reported in the literature, application of the Ni(II) complex in carboxylation of unsaturated compounds has three advantages: (1) the selectivity in the production of cinnamic acid; (2) more increase in the reduction current indicating that the carboxylation of phenylacetylene is fast; and (3) the potential shift of electrocatalytic reduction of CO₂ to less negative values showing that the Ni(II) complex has an excellent electrocatalytic activity for CO₂ reduction. According to the voltammetric and coulometric results, an EC′CCC′C mechanism was proposed for the electrocatalytic synthesis of cinnamic acid.