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Electrocatalytic acetylene hydrogenation to ethylene (E‐AHE) is a promising alternative for thermal‐catalytic process, yet it suffers from low current densities and efficiency. Here, we achieved a 71.2 % Faradaic efficiency (FE) of E‐AHE at a large partial current density of 1.0 A cm−2 using concentrated seawater as an electrolyte, which can be recycled from the brine waste (0.96 M NaCl) of alkaline seawater electrolysis (ASE). Mechanistic studies unveiled that cation of concentrated seawater dynamically prompted unsaturated interfacial water dissociation to provide protons for enhanced E‐AHE. As a result, compared with freshwater, a twofold increase of FE of E‐AHE was achieved on concentrated seawater‐based electrolysis. We also demonstrated an integrated system of ASE and E‐AHE for hydrogen and ethylene production, in which the obtained brine output from ASE was directly fed into E‐AHE process without any further treatment for continuously cyclic operations. This innovative system delivered outstanding FE and selectivity of ethylene surpassed 97.0 % and 97.5 % across wide‐industrial current density range (≤ 0.6 A cm−2), respectively. This work provides a significant advance of electrocatalytic ethylene production coupling with brine refining of seawater electrolysis. [ABSTRACT FROM AUTHOR]