It is of significance to develop an active, efficient electrocatalyst for the oxygen evolution reaction (OER) as this determines the efficiency and cost of water/seawater electrolysis. Here, a cost-effective Ni─Fe hydroxide as a promising OER catalyst is developed by 1 min ultrafast method. The catalyst shows low OER overpotentials of 240 and 254 mV at 10 mA cm in both 1 m KOH and alkaline seawater, respectively. It also exhibits excellent electrochemical stability. In situ Raman spectra and other physical characterizations prove the incorporation of Fe and the transformation of Ni(Fe)(OH) to Ni(Fe)OOH are responsible for the enhancement of the OER performance. Furthermore, the Ni─Fe hydroxide catalyst can be readily scaled up and synthesized within 1 min. The catalyst with a size of 2000 cm still remains electrochemically uniform. The alkaline electrolysis cell integrated with the Ni─Fe catalyst as the anode and commercialized porous NiMo foam as the cathode has demonstrated a current density of 200 mA cm at 2.3 and 2.9 V in 6 m KOH and alkaline seawater at 60 °C, respectively. Therefore, the ultrafast synthesized, earth-abundant Ni─Fe catalyst is scalable, economical, and highly active for OER, which is promising for industrial water/seawater splitting applications.