Background/Objectives: Radiotherapy is a cornerstone of head and neck cancer treatment, but it can adversely affect periodontal tissues by impairing vascularity, cellularity, and healing capacity. The present study aimed to histologically and immunohistochemically evaluate radiotherapy-induced changes in periodontal tissues in a rat model of experimental periodontitis, with particular focus on periodontal ligament width (PerioW) and the RANK/RANKL/OPG axis at different healing stages. Methods: Seventy-two male Sprague–Dawley rats were allocated to three groups: irradiation only (Rt), ligature-induced periodontitis only (Pt), and ligature-induced periodontitis followed by irradiation (PtRt). Experimental periodontitis was induced by placing ligatures around the maxillary first molars for two weeks. On the day of ligature removal, the irradiated groups received a single 20 Gy dose to the head and neck region. Animals were euthanized on days 1, 15, and 30 after ligature removal (n = 8/group/time point). Histomorphometric analysis of PerioW was performed on H&E-stained sections, and immunohistochemical staining was used to quantify RANK, RANKL, and OPG expression. Results: On day 1, PerioW did not differ significantly among groups, although the PtRt group had the highest mean. A significant intergroup difference was observed for RANKL, with higher expression in PtRt than in Pt. On day 15, PerioW differed significantly among groups, with the lowest value in Rt and the highest in PtRt; the Rt–PtRt comparison was significant. At this time point, RANK, RANKL, OPG, and the RANKL/OPG ratio showed no significant intergroup differences. On day 30, no significant intergroup differences were found for PerioW or immunohistochemical parameters; however, PtRt continued to show the highest PerioW and OPG values and the lowest RANKL/OPG ratio. Conclusions: Radiotherapy superimposed on periodontitis enhanced early pro-resorptive signaling and delayed structural normalization of periodontal tissues. Although late increases in OPG suggested a compensatory response, this appeared insufficient to fully reverse radiation-associated periodontal alterations. These findings support the importance of controlling periodontal inflammation before radiotherapy to reduce subsequent periodontal tissue damage.