Because of its importance in enhancing charge separation and transfer, built-in electric field engineering has been acknowledged as an effective technique for improving photocatalytic performance. Herein, a stable p–n heterojunction of 2D/2D (2D: two-dimensional) Co₃O₄/ZnIn₂S₄ with a strong built-in electric field is precisely constructed. The Co₃O₄/ZnIn₂S₄ heterojunction exhibits a higher visible-light photocatalytic hydrogen (H₂) evolution rate than the individual components, which is primarily attributed to the synergy effect of improved light absorption, abundant active sites, short charge transport distance, and high separation efficiency of photogenerated carriers. Furthermore, the photoelectrochemical studies and density functional theory (DFT) calculation results demonstrate that the enhanced interfacial charge separation and migration induced by the generated built-in electric field are the critical reasons for the boosted photocatalytic performance. This research might pave the way for the rational design and manufacturing of 2D/2D heterojunction photocatalysts with extremely efficient photocatalytic performance for solar energy conversion.