Elucidating the site symmetry of lanthanide dopants in micro/nanocrystals is essential for a comprehensive understanding of upconversion luminescence (UCL). Here, we report an excitation-wavelength-dependent polarization response in individual $\mathrm{E}{\mathrm{r}}^{3+}$-doped $\beta \text{−}\mathrm{NaY}{\mathrm{F}}_4$ microrods, where the degree of excitation polarization is tunable from 0 to 1 and the polarization orientation can be rotated by π/2. Further investigations reveal that subsequent excited-state absorption inherits the polarization orientation established by the initial ground-state absorption, with the degree of excitation polarization increasing as more excited-state absorption steps are involved in the upconversion process. Based on the Stark emission peaks of the ${}^4\mathrm{S}_{3/2}$ multiplet, polarization-verified crystal field calculations indicate that $\mathrm{E}{\mathrm{r}}^{3+}$ occupies spectroscopic sites with approximately ${\mathrm{C}}_3$ symmetry in the host. By assigning irreducible representations to all Stark levels, we establish a correlation between the macroscopic polarized UCL properties of $\mathrm{E}{\mathrm{r}}^{3+}$ and its local symmetry in a $\beta \text{−}\mathrm{NaY}{\mathrm{F}}_4$ lattice via point-group selection rules. Our work provides valuable insights into the polarization origins of UCL, offering an approach to optimize upconversion and realize diverse applications in polarized photonics.

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• Crystal symmetry
• Frequency conversion
• Polarization of light
• Stark effect
• Rare-earth doped crystals
• Rare-earth ions
• Crystal-field theory
• Polarization-dependent photoemission spectroscopy