Interfacial Magnetic Order Controls Valley Polarization in WS₂/Fe₃GaTe₂ Heterostructures

van der Waals heterostructures integrating two-dimensional magnets with transition-metal dichalcogenides provide a promising platform for controlling coupled spin and valley physics, yet the origin of magnetically induced valley polarization remains unresolved. Here we show that interfacial magnetic order governs valley exciton dynamics in WS₂/Fe₃GaTe₂ heterostructures. By comparing bottom- and top-stacked architectures, we disentangle intrinsic magnetic proximity effects from oxidation-driven interfacial reconstruction. The protected bottom-stacked interface preserves ferromagnetic order, yielding pronounced valley hysteresis and a giant valley exciton Zeeman splitting of 13.7 meV, equivalent to an effective magnetic field of ∼59 T. In contrast, the exposed top-stacked interface exhibits only 1.26 meV splitting. Polarization-resolved photoluminescence, high-field magneto-optical measurements, and first-principles calculations reveal that oxidation induces an interfacial transition from ferromagnetic to antiferromagnetic order, suppressing the proximity exchange field through weakened Fe-W orbital hybridization. These results establish interfacial magnetic reconstruction as the key factor governing magneto-valley coupling.