  We present a theory of an optically induced valley polarization in an
interacting, time-reversal symmetric Weyl semimetal placed under strong
magnetic fields. Because the application of a magnetic field reduces the
symmetry of the crystal, the optical absorption intensity differs at Weyl nodes
that were equivalent by symmetry at zero field. At strong magnetic field, the
difference in the absorption intensity reaches 100% for a sizeable frequency
interval of the incident light. This complete valley polarization originates
from interband transitions involving the chiral Landau level, and can be
controlled by changing the directions of the magnetic field and the light
propagation. We identify the splitting of $0\to 1$ or $-1\to 0$ inter Landau
level transitions as an observable signature of the complete valley
polarization, and discuss its manifestation in the TaAs family of materials.
