Halide ions are naturally abundant in oceans and estuaries. Large amounts of highly saline efflux are also made and discharged to surface water from desalination processes and from unconventional oil and gas recovery. These highly concentrated halides can generate reactive halogen radicals. However, the redox reactions of halogen radicals with heavy metals or transition metals have received little attention. Here, we report undiscovered fast oxidation of manganese ions (Mn²⁺) by reactive halogen radicals. Hydroxyl radicals (˙OH) are produced by nitrate photolysis. While ˙OH radicals play a limited role in the direct oxidation of Mn²⁺, ˙OH can react with halide ions to generate reactive halogen radicals to oxidize Mn²⁺. In addition, more Mn²⁺ was oxidized by bromide (Br) radicals generated from 1 mM Br⁻ than by chloride (Cl) radicals generated from 500 mM Cl⁻. In the presence of Br radicals, the abiotic oxidation rate of Mn²⁺ to Mn(IV)O₂ nanosheets is greatly promoted, showing a 62% increase in Mn²⁺(aq) oxidation within 6 h of reaction. This study advances our understanding of natural Mn²⁺ oxidation processes and highlights unexpected impacts of reactive halogen radicals on redox activities with heavy metals and corresponding nanoscale solid mineral formation in brine. This work suggests a new, environmentally-friendly, and facile pathway for synthesizing MnO₂ nanosheets.