RuO₂ shows high acidic OER activity as Ir-free catalysts, but this feature also induces lattice-oxygen overactivation, Ru overoxidation, dissolution, and irreversible reconstruction and consequent limited stability. Heterojunctions engineering has emerged as an effective strategy to improve acidic OER catalysts by coupling chemically distinct components besides doping. However, previous studies have mainly emphasized heterostructure construction and performance enhancement, while the atom-level interfacial functions that govern acidic OER activity and stability have been somewhat overlooked. In this review, RuO₂-based heterojunctions are proposed as functional interfacial reactors rather than simple multicomponent composites for acidic OER. Heterointerfaces regulate acidic oxygen evolution through five interconnected functions: electronic coupling, oxygen spillover and redox buffering, interfacial water/proton microenvironment regulation, dual-site cooperation, and balanced lattice-oxygen redox. These functions are individually elucidated using representative Ru-based heterojunctions, including Ru/RuO₂, RuO₂/LiCoO₂, ZrO₂-RuO₂, RuO₂/MoO₃, and RuO₂-CeO₂, to reveal how specific interfacial structures regulate charge redistribution, Ru-O covalency, oxygen-species migration, water/proton dynamics, and cooperative O-O bond formation, thereby enhancing OER activity while suppressing Ru overoxidation and dissolution. This review should also be inspiring for designing Core/shell or loaded RuO₂-based catalysts, and thus provides fundamental understanding for designing durable, low-noble-metal acidic OER catalysts for PEMWE.