  Magnetic-field control of fundamental optical properties is a crucial
challenge in the engineering of multifunctional microdevices. Van der Waals
(vdW) magnets retaining a magnetic order even in atomically thin layers, offer
a promising platform for hosting exotic magneto-optical functionalities owing
to their strong spin-charge coupling. Here, we demonstrate that a giant optical
anisotropy can be controlled by magnetic fields in the vdW magnet FePS$_3$. The
giant linear dichroism ($\sim$11%), observed below $T_{\text{N}}\!\sim\!120$ K,
is nearly fully suppressed in a wide energy range from 1.6 to 2.0 eV, following
the collapse of the zigzag magnetic order above 40 T. This remarkable
phenomenon can be explained as a result of symmetry changes due to the spin
order, enabling minority electrons of Fe$^{2+}$ to hop in a honeycomb lattice.
The modification of spin-order symmetry by external fields provides a novel
route for controllable anisotropic optical micro-devices.
