The current thermocatalytic acetylene hydrogenation process suffers from the use of excessive hydrogen and the noble metal Pd, high temperatures and overhydrogenation. Here we report an electrocatalytic semihydrogenation strategy to selectively reduce acetylene impurities to ethylene under ambient conditions. For a crude ethylene flow that contains 1 × 10⁴ ppm acetylene, electrochemically deposited Cu dendrites exhibited a high specific selectivity of 97%, continuous production of a polymer-grade ethylene stream (4 ppm acetylene) at a large space velocity of 9.6 × 10⁴ ml g_cat^–1 h^–1 and excellent long-term stability. Theoretical and operando electrochemical Raman investigations revealed that the outstanding electrocatalytic acetylene semihydrogenation performance of Cu catalysts originates from its exothermic acetylene adsorption and ethylene desorption. Meanwhile, the electrocatalytic semihydrogenation strategy is universally applicable for hydrogenating other alkyne impurities to produce polymer-grade olefins, for example, propylene and 1,3-butadiene. [Figure not available: see fulltext.]