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Heterostructures with abundant phase boundaries are compelling for surface-mediated electrochemical applications. However, rational design of such bifunctional electrocatalysts for efficient hydrogen and oxygen evolution reactions (HER and OER) is still challenging. Here, due to the well-matched lattice parameters, we easily achieved the epitaxy of two-dimensional ternary nickel thiophosphate (NiPS₃) nanosheets with in-grown dinickel phosphide (Ni₂P) through an in situ growth strategy. Density functional theory calculations reveal that the NiPS₃/Ni₂P heterojunction significantly decreases the kinetic barrier for hydrogen adsorption and accelerates electron transfer due to the built-in electric field at the epitaxial interfaces. The significantly improved electrocatalytic performance is shown to be closely related to the epitaxial interfacial area rather than the amount of secondary phase. Notably, the resultant NiPS₃ …