Electrocatalytic seawater splitting is an eco-friendly approach for high-purity hydrogen production, but still hampered by anodic oxygen evolution reaction (OER). Besides the exploitation of efficient electrocatalysts, integrating solar energy into electrocatalytic system can offer a complementary degree of freedom to modulate such electrochemical process. Herein, we construct the hierarchically structured S-doped NiFe hydroxide/oxide on carbon cloth (S-NiFeO x H y /CC) that exhibits both intrinsically good OER activity and solar light absorption capability for solar-assisted OER in seawater splitting. When exposed to solar light, the S-NiFeO x H y /CC delivers a current density of 10 mA cm−2 at overpotential of 250 mV with a low Tafel slope of 63 mV dec−1 and good stability in alkaline simulated seawater. This work affords a promising strategy to intensify the electrocatalytic reaction by solar energy for seawater electrolysis, which has important implications for efficient hydrogen production. [Display omitted] • S-doped NiFe hydroxide/oxide electrode with hierarchical nanostructure is applied for OER in seawater splitting. • S-NiFeO x H y /CC possesses electrocatalytic activity, photoresponse capability and corrosion resistance. • S-NiFeO x H y /CC assisted by solar energy exhibits a overpotential of 10 = 250 mV with a Tafel slope of 63 mV dec−1. • The auxiliary role of solar energy is ascribed to the phonon-photon synergistic promotion effect. [ABSTRACT FROM AUTHOR]