Exploitation　of　atomic-level　principles　to　optimize　the　charge　transfer　on　ultrathin　2D　heterostructures　is　an　emerging　frontier　in　relieving　the　energy　and　environmental　crisis.　Herein,　a　facile　“topological-atom-extraction”　protocol　is　disclosed,　i.e.,　selective　extraction　of　Zn　from　ultrathin　half-unit-cell　ZnIn2S4　(HZIS)　can　embed　thin　In2O3　domain　into　1.60 nm　thick　HZIS　layer　to　create　an　atomically　thin　in-plane　In2O3/HZIS　heterostructure.　Thanks　to　the　optimal　distance　and　capability　of　charge　separation,　the　in-plane　In2O3/HZIS　heterostructure　is　among　the　best　ZnIn2S4-based　CO2　reduction　reaction　(CRR)　photocatalysts,　and　indeed　demonstrates　a　significant　increase　(from　6.8-　to　128-fold)　in　CO　production　rate　compared　with　those　of　out-plane　ZIS@In2O3　and　out-plane　In2O3-HZIScalcined　heterostructures.　Density　Functional　Theory　simulation　reveals　that　whereas　the　out-plane　heterostructure　has　a　much　smaller　∆q　of　0.2–0.25 e,　the　in-plane　heterostructure　with　“zero　distance　contact”　has　an　optimal　∆q　of　1.05 e　between　In2O3　and　HZIS　that　induces　remarkable　charge　redistribution　on　the　in-plane　heterojunction　interface　and　creates　local　electric　field　confined　within　the　ultrathin　layer.　The　charge　redistribution　efficiently　directs　the　charge-carrier　separation　in　S-scheme　photocatalytic　system　and　endows　long-lifetime　carrier　to　CRR　active　HZIS.　The　findings　demonstrate　the　strong　versatility　of　engineering　atomic-level　heterojunctions　for　efficient　catalysts　design.　©　2022　Wiley-VCH　GmbH.