This study explored the impact of western diet (WD) on a ligature-induced periodontitis (PD) model. After either control diet (CD) or WD feeding for 16 weeks, male Wistar rats were allocated in six groups (n = 6). The first and second groups had no PD. The third and fourth groups had ligature-induced PD for 10 days, while the fifth and sixth groups had ligature-induced PD for 10 days, followed by ligature removal healing period for another 10 days. The CD contained 13.71% protein, 75.98% carbohydrate, and 10.31% fat, though WD composed of 14.7% protein, 40.7% carbohydrate, and 44.6% fat. After clinical evaluation, the maxillary alveolar bone and gingival tissues were collected for morphometric, microstructural, histological, and gene expression analyses. There were significant increases in the gingival bleeding index, periodontal probing depth, and tooth mobility in WD animals with PD and in the healing groups. The WD groups had a greater alveolar crest height, indicating greater bone resorption. Disruption of the bone microarchitecture by PD was exacerbated in WD-fed animals. The histological evaluation demonstrated a greater extent of gingival inflammation in the PD groups. The Tnf, Il6, Ctsk, and Tnfsf11/RANKL gene expression levels were increased in the WD groups, while the Bglap and Hif1a gene expression levels were decreased in the WD groups. Findings of the study are compelling preclinical evidence that WD deteriorates periodontal health and exacerbates periodontal disease and alveolar bone loss in experimental animals. Future clinical research is warranted to translate these preclinical findings.