Operating Zn‖MnO$_2$ and Zn‖PbO$_2$ batteries in highly acidic electrolytes enables much higher output voltages (up to ca. 2 V and 2.5 V, respectively) than near-neutral systems, making them attractive for high-energy aqueous zinc-ion batteries. However, Zn electrodes suffer from severe hydrogen evolution reaction (HER) and self-corrosion in acidic media, limiting their cycling stability. Here, we introduce indium ions to stabilize Zn electrodes in highly acidic electrolytes. During Zn plating, indium accumulates at the electrode surface to form a protective In–Zn alloy layer, which alleviates the HER by weakening the hydrated proton adsorption and promotes preferential growth of the Zn (002) facet, enabling highly reversible Zn plating/stripping. Moreover, the replacement deposition during rest can generate an indium-containing protection layer that mitigates Zn self-corrosion. Consequently, Zn‖Zn symmetric cells exhibit an extended cycling life approaching 3000 h (at pH 1.13), while acidic Zn‖MnO$_2$ coin cells deliver a record-high discharge plateau of ∼1.95 V with stable cycling over 1100 cycles. Notably, the effectiveness of In$^{3+}$ additives is validated in highly acidic Zn‖PbO$_2$ batteries, demonstrating their general applicability. This work provides a comprehensive understanding of the In$^{3+}$-regulated interfacial chemistry for high-voltage Zn batteries in highly acidic electrolytes.