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[Display omitted] • A two-dimensional nanosheets NiCoFeTa-based OER electrocatalyst was fabricated. • The Ta element accelerated the formation of the catalytically active phase γ-NiOOH, effectively achieving a stable catalytic active phase. • The NiCoFeTa-OH sample shows a low overpotential of 330 mV for OER under 500 mA cm-2. • The seawater electrolyzer can maintain 800 h under a current density of 1 A cm-2. Considering the regional distribution of freshwater resources, acquiring hydrogen by seawater electrolysis has lower cost and feasibility. NiCoFe-based materials have been widely studied due to their excellent catalytic oxidation performance in freshwater. However, the large amount of chloride ions in seawater will severely erode the catalytic site and greatly reduce the catalyst's lifetime. In this paper, a two-dimensional nanosheet quaternary metal hydroxide electrode composed of Ni, Co, Fe, and Ta elements was synthesized via a facile one-step hydrothermal method for efficient oxygen evolution reaction (OER) in seawater electrolysis. Structural characterization and in-situ test revealed that the incorporation of Ta not only optimized the electronic structure of the catalyst but also accelerated the formation of the γ-NiOOH phase, making the active phase is protected from being attacked by Cl- ions during the formation process, enhancing its OER activity while achieving extended durability. The catalyst was operated at an ampere-level of 1 A cm-2 for over one month (800 h), and no significant degradation was observed. This work provides a promising strategy for developing cost-effective, efficient, and stable catalysts for challenging applications in seawater electrolysis. [ABSTRACT FROM AUTHOR]