Background: Gastroesophageal reflux disease (GERD) and periodontitis are common conditions in older adults that can lead to serious complications, gaining public health attention. Although GERD and periodontitis share common risk factors, such as age, lifestyle habits, and socioeconomic status, large-scale studies examining their specific relationship are limited. Methods: This study aimed to assess the association between GERD and the likelihood of developing periodontitis using a national validated cohort data from Korea. Data were drawn from the Korean National Health Insurance Service Health Screening Cohort (2004–2019) using the International Classification of Diseases, 10th Revision, Clinical Modification (ICD-10-CM) diagnostic codes. The study included 16,744 GERD patients and 66,976 matched controls (matched 1:4 by age, sex, income, and residence). Cox proportional hazard models estimated hazard ratios (HRs) for periodontitis, adjusting for various factors, with subgroup analyses based on age, income, and other covariates. Results: The overall incidence of periodontitis was similar between the GERD and control groups, with no significant difference in the adjusted hazard ratios (aHR = 1.00, 95% CI 0.99–1.02, p = 0.625). However, subgroup analyses in the GERD group showed a slightly increased likelihood of periodontitis in individuals over 60 years old (aHR = 1.03, 95% CI 1.00–1.06, p = 0.050) and those in lower-income brackets (aHR = 1.03, 95% CI 1.01–1.06, p = 0.023). Conclusions: In summary, while GERD was not associated with an increased overall probability of periodontitis, age and socioeconomic factors may influence the likelihood of periodontitis development in certain GERD patients. These findings may highlight the need for targeted preventive strategies and closer oral health monitoring in older and lower-income populations with GERD.