Transition-metal doping enables the introduction of spin functionality into halide double perovskites, while simultaneously modifying optical properties. Here, we combine controlled single-crystal growth, optical characterization, comprehensive electron paramagnetic resonance (EPR) spectroscopy, and first-principles modeling to identify the microscopic nature of Fe-related centers in Fe-doped ${\mathrm{Cs}}_2{\mathrm{AgBiBr}}_6$. Single crystals with nominal $\mathrm{F}{\mathrm{e}}^{3+}$ concentrations up to 15% in the precursor stage were grown using a controlled-cooling method, yielding reproducible Fe incorporation up to 0.1% with respect to Bi, without secondary phases. Despite this low concentration, Fe doping introduces electronic states that influence optical absorption and photoluminescence. EPR measurements reveal an S = 5/2 $\mathrm{F}{\mathrm{e}}^{3+}$-related center whose anisotropy follows the cubic-to-tetragonal phase transition below 120 K. Angular-dependent EPR resolves two configurations of this nearly axial spin center, with principal axes rotated by 90° and aligned with the a/b plane of the tetragonal lattice. Density-functional calculations attribute these centers to impurity-vacancy complexes, most likely ${\mathrm{Fe}}_{\mathrm{Bi}}\text{−}{\mathrm{V}}_{\mathrm{Br}}$, that stabilize in a basal configuration of the low-temperature phase. This approach resolves vacancy-coupled defect orientations, narrowing possible models to $\mathrm{F}{\mathrm{e}}^{3+}$-vacancy complexes and establishing them as stable, orientation-sensitive spin probes of structural symmetry in halide double perovskites, while providing a microscopic basis for tuning their magnetic and optical responses.

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• Magnetic interactions
• Magnetic semiconductors
• Paramagnets
• Crystal growth
• Density functional theory
• Electron paramagnetic resonance
• Photoluminescence