<p>The correlation between crystal structure and luminescence properties in rare-earth-doped materials remains a key issue for the design of advanced phosphors. In this work, Er³⁺-doped GdP₅O₁₄ ultraphosphates (Gd_1−xEr_xP₅O₁₄, 0.05 ≤ x ≤ 0.40) were synthesized via a conventional solid-state reaction. Structural characterization by X-ray diffraction combined with Rietveld refinement reveals the coexistence of two monoclinic polymorphs (P2₁/c and C2/c) at low and intermediate Er³⁺ concentrations, followed by a progressive stabilization of the C2/c phase with increasing dopant content. A complete structural transformation into a single C2/c phase is achieved at x = 0.35, indicating the formation of a homogeneous solid solution. Low-temperature (10&#xa0;K) photoluminescence spectroscopy under 488&#xa0;nm excitation provides direct evidence of the strong interplay between local structure and optical response. The emission spectra highlight the presence of three distinct Er³⁺ luminescent sites in the 0.05 ≤ x ≤ 0.30 range, associated with one low-symmetry C_i site in the P2₁/c phase and two C_ii sites in the C2/c phase, in excellent agreement with crystallographic findings. The evolution of spectral features with increasing Er³⁺ concentration reflects both site redistribution and enhanced crystal field effects. These results demonstrate that Er³⁺ ions act not only as efficient optical emitters but also as sensitive probe of the local structural environment. This study establishes a clear structure–luminescence correlation in GdP₅O₁₄ ultraphosphates and provides valuable insights for tuning the optical properties of rare-earth-based phosphors through controlled dopant concentration. The observed structure-dependent luminescence behavior highlights the potential of these materials for applications in solid-state lighting, optical sensing, display technologies, and photonic devices requiring tailored emission characteristics.</p>