Selective separation of bromine (Br₂) from iodine (I₂) remains a fundamental bottleneck in industrial halogen separation and applied bromination chemistry. A safe, efficient, and selective Br₂ supply is indispensable for large-scale transformations and product purity, yet separation is complicated by their comparable kinetic diameters (3.5 and 4.0 Å, respectively) and strong tendency to form interhalogen species. Conventional capture technologies remain hindered by limited stability, low capacity, and poor selectivity. Here, structurally tunable squaramine-based [2+2] macrocycles with intrinsic cavity diameters (∼3.5 Å) are introduced as efficient and selective molecular sieves. The pristine squaramine [2+2] macrocycle achieves high Br₂ uptake of 4.9 g.g⁻¹ from cyclohexane solution and 5.7 g.g⁻¹ in the vapor phase, while its hydrochloride derivative ([2+2].HCl) displays substantial Br₂ uptake (4.3 g.g⁻¹ cyclohexane, 3.0 g.g⁻¹ in vapor) and completely excludes I₂ even under concentrated mixed-halogen conditions. Single-crystal X-ray diffraction revealed a chloride-to-bromide exchange mechanism stabilizing unique polybromide species, further supported by UV–vis, XPS, Raman spectroscopy, and metadynamics simulations. These findings establish squaramine [2+2] as the first macrocyclic molecular sieves for selective Br₂ capture and total I₂ rejection, offering robust and scalable platforms for halogen separation, environmental remediation, and safe, easy-to-handle Br₂ reservoirs for selective catalytic and synthetic bromination processes.