The investigation of cost-effective and highly efficient electrocatalysts for alkaline water and simulative seawater oxidation is essential to the conversion and storage of renewable energy. In this paper, the Ce–NiSe2/CoP catalyst with multihole and ultrathin nanosheet structure is generated. The unique structural characteristics of Ce–NiSe2/CoP heterojunction nanosheets contribute the excellent OER performance under different alkaline 1 M KOH and simulated seawater (1 M KOH + 0.5 M NaCl) electrolytes. Specifically, this catalyst exhucture is generated. The unique structural characteristics of Ce–NiSe2/CoP heterojunction nanosheets conibits the low overpotentials of 287 and 304 mV at the current density of 10 mA cm−2, along with the Tafel slopes of 87.1 and 78.8 mV dec−1 in two solutions, respectively. Moreover, the Ce–NiSe2/CoP target product also displays good stability. The present study introduces a promising strategy for the advancement of high-performance electrocatalysts in green energy field. Display omitted •The Ce-doped NiSe2/CoP heterostructure nanosheets are triumphantly generated.•The Ce–NiSe2/CoP product possesses multihole and ultrathin lamella architecture.•Each nanosheet with coarse surface is composed of prolifically nanosecondary units.•This catalyst presents fine OER properties in alkaline water and simulative seawater.