A Ni-BDC/NH₂-MIL-88B(Fe) heterojunction (Ni-BDC/NM88B(Fe)) was synthesized as an efficient oxygen evolution reaction (OER) electrocatalyst for seawater electrolysis. This MOF-on-MOF structure exhibits high stability (200 h) and low overpotentials (232 and 299 mV at 100 mA cm⁻² in alkaline freshwater and seawater, respectively). The exceptional performance is attributed to the rapid self-reconstruction of Ni-BDC/NM88B(Fe) to form a NiFeOOH protective layer, preventing chloride-induced dissolution. The interface interaction between Ni-BDC and NM88B(Fe) forms Ni–O–Fe bonds, promoting electron transfer and lowering the energy barrier of the rate-determining step, thus accelerating OER. The catalyst shows excellent OER activity and stability in seawater, with a low overpotential of 299 mV at 100 mA cm⁻². The Ni-BDC/NM88B(Fe) heterojunction enhances electron transport and provides a protective layer against chloride corrosion. The unique structure also facilitates OER by promoting OH⁻ adsorption and reducing the energy barrier. The catalyst demonstrates superior performance compared to commercial materials, with high activity and durability in seawater. The study highlights the potential of MOF-on-MOF heterojunctions for efficient seawater electrolysis.A Ni-BDC/NH₂-MIL-88B(Fe) heterojunction (Ni-BDC/NM88B(Fe)) was synthesized as an efficient oxygen evolution reaction (OER) electrocatalyst for seawater electrolysis. This MOF-on-MOF structure exhibits high stability (200 h) and low overpotentials (232 and 299 mV at 100 mA cm⁻² in alkaline freshwater and seawater, respectively). The exceptional performance is attributed to the rapid self-reconstruction of Ni-BDC/NM88B(Fe) to form a NiFeOOH protective layer, preventing chloride-induced dissolution. The interface interaction between Ni-BDC and NM88B(Fe) forms Ni–O–Fe bonds, promoting electron transfer and lowering the energy barrier of the rate-determining step, thus accelerating OER. The catalyst shows excellent OER activity and stability in seawater, with a low overpotential of 299 mV at 100 mA cm⁻². The Ni-BDC/NM88B(Fe) heterojunction enhances electron transport and provides a protective layer against chloride corrosion. The unique structure also facilitates OER by promoting OH⁻ adsorption and reducing the energy barrier. The catalyst demonstrates superior performance compared to commercial materials, with high activity and durability in seawater. The study highlights the potential of MOF-on-MOF heterojunctions for efficient seawater electrolysis.