Malachite green (Basic Green 4), a persistent cationic triphenylmethane dye common in textile and aquaculture effluents, poses environmental risks due to its poor biodegradability and toxicity. This study aims to determine how halide counterions (Cl⁻ vs. Br⁻) influence the selectivity and mechanisms of its oxidative degradation in aqueous solution at the molecular level. Density functional theory (M06-2X/6-31++G(d,p)) calculations were performed to analyze electronic structure, solvation entropy, and Fukui reactivity indices. Quantum-chemical results show that halide identity modulates dye reactivity through differences in orbital stabilization, electrophilicity, and polarity. Cl⁻ pairing enhances entropic stabilization and facilitates charge redistribution, concentrating electrophilic regions for effective hydration, while Br⁻ promotes hybrid radical/ionic degradation pathways due to its polarizability. Fukui indices identify aromatic ring carbons as the most reactive sites for hydroxyl radical attack. Distinct pathways are predicted: MG–Cl⁻ undergoes single-electron transfer followed by nucleophilic ring scission, whereas MG–Br⁻ proceeds through mixed radical–ionic transformations. These findings demonstrate that halide counterions govern degradation selectivity via solvation and ion-pairing dynamics, providing a molecular-level framework for designing more efficient halide-tuned advanced oxidation processes for recalcitrant dye removal.