The electronic properties of monolayer WTe₂ on top of Fe₃O₄(111) are investigated by density functional theory. We find that the substrate termination of Fe₃O₄(111) can switch the conductivity of monolayer WTe₂ from the p- to n-type. However, the stacking pattern can critically influence its electronic structure. For Fe(A)-terminated interfaces, stronger-bonding models show Fermi level pinning. Additionally, the time-reversal symmetry is broken by the proximity that leads to valley polarization. With particular stacking patterns, large valley splittings of 139, −76 and −72 meV are obtained for Fe(A)-, Fe(B)- and O-terminated models, respectively. Moreover, Fe(B)- and O-terminated ones have more applicable significance for valleytronics as no interference of the interface state appears at the valence band maximum. We demonstrate that proximity to a room-temperature ferromagnet is a convenient way to obtain valley polarization and adjust the conductivity of monolayer WTe₂.