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This report is on experiments and theory on the process of optically stimulated electron population density redistribution in Si-substituted yttrium-iron garnet single crystals at 77 K. It was determined that a photo-induced uniaxial anisotropy field arose in the YIG:Si sample in response to illumination by quasi-linearly polarized laser (λ = 808 nm) leading to redistribution of Fe2+ ions among the nonequivalent octahedral sites. The photo-induced field was measured by variation of ferromagnetic resonance (FMR) frequencies in the X-band. The measured FMR frequency shift demonstrated a pronounced dependence on the polarization vector orientation with respect to crystallographic axes, in accordance with the theory discussed here. The frequency shift dependence on light intensity (for optimal polarization orientation) was found to be nearly linear, at least within the output intensity range of the optical source. The maximum frequency shift was −130 MHz for 75 mW applied optical power. A similar phenomenon was also observed at room temperature but was attributed to the sample heating by the incident light. The results presented here demonstrate the potential of the phenomenon for application in the development of ferrite signal processing devices with dual tuning by both magnetic field and optical irradiation.