Background <p>With global shifts in diet and environment, understanding how dietary factors and inflammation affect oral diseases like periodontitis is essential for prevention. The Planetary Health Diet Index (PHDI) promotes a plant-based diet for both health and sustainability, while the Dietary Inflammatory Index (DII) measures a diet’s inflammatory potential. This study examines the link between PHDI and periodontitis and assesses DII’s moderating role.</p> Methods <p>This study conducted a cross-sectional analysis of 9,016 participants from the 2009–2014 NHANES database. We used subgroup analysis, restricted cubic splines (RCS), and multivariable logistic regression to investigate the association between PHDI and periodontitis prevalence. Additionally, mediation analysis was performed to explore the potential relationship between DII, PHDI, and periodontitis.</p> Results <p>This study included 9,016 participants, of whom 4,521 were diagnosed with periodontitis. After adjusting for potential confounders in multivariable logistic regression models, a 10-point increase in PHDI was associated with a 6% lower prevalence of periodontitis (odds ratio [OR] = 0.94, 95% confidence interval [CI]: 0.89–0.99). Conversely, each one-unit increase in DII was associated with a 6% higher prevalence of periodontitis (OR = 1.06, 95% CI: 1.01–1.10). These associations remained robust when PHDI and DII were categorized into tertiles. RCS analysis demonstrated a significant linear relationship between PHDI and periodontitis. Mediation analysis indicated that DII mediated 21.74% of the association between PHDI and periodontitis (<i>P</i> = 0.004).</p> Conclusions <p>The results of this study show a strong negative correlation between PHDI and the prevalence of periodontitis. Additionally, DII appears to mediate this relationship, suggesting that an anti-inflammatory diet may offer benefits.</p>