Control of the stability, transport, and confinement of charge carriers (electrons and holes) at interfaces is a key requirement to realize robust halide perovskite devices. The PbS–CsPbBr3 interface is atomically matched with low lattice strain, opening the potential for epitaxial growth. We assess the atomic nature of the interface using first-principles density functional theory calculations to identify (1) the thermodynamically stable (100) surface termination of the halide perovskite; (2) the most favorable (100)|(100) contact geometry; (3) the strong interfacial chemical bonding between PbS and CsPbBr3; (4) the type I (straddling) band alignment that enables electron and hole confinement in the lead sulfide layer. The combination of metal halide perovskites and IV–VI semiconductors represents an important platform for probing interfacial chemical processes and realizing new functionality.