Cost-effective oxygen evolution reaction (OER) catalysts with both superb activity and stability are crucial for practical electrocatalytic water splitting. We herein come up with a metal carbonate hydroxide and metal sulfide (MS/MCH) heterostructure via in situ generating MS on MCH to tackle the conflict between activity and durability. The heterostructure in a unique “nanoparticle-in-nanosheet” configuration features abundant hetero-interfaces between MS and MCH, allowing for effective electron transfer from the antibonding orbital of M–S in MS to M–O in MCH and thus improving the stability of sulfides during water oxidation. Simultaneously, such coupling is able to modulate the electronic structure of the metal centre, thus enhancing the intrinsic activity. The coupled heterostructure therefore exhibits steady OER performance with an ultrasmall overpotential of 226 to deliver a current of 10 mA cm⁻² and can output an industrial-level current of 1000 mA cm⁻² at an overpotential of only 367 mV. Furthermore, the overall water electrolyzer with such a heterostructure as an anode presents stable hydrogen production at 100 mA cm⁻² with a small cell voltage of 1.62 V and a high solar to hydrogen efficiency of 16.99%. Such superior catalytic performance demonstrates the feasibility of introducing a heterostructure to tailor the local electrocatalytic properties and opens up an avenue toward designing efficient OER catalysts for clean hydrogen production and diverse energy conversion devices.