Periodontitis is a common chronic inflammatory disease which could have an important impact on blood pressure (BP). This study aimed to explore (a) the association between periodontal health and BP in a large representative cohort, (b) the predictive value of diagnosis of periodontitis in undiagnosed raised BP and (c) whether age is a mediator of this relationship. In total, 1057 randomly recruited individuals (mean age, 60.9 ± 16.3 years, 57.7% women) underwent periodontal clinical assessment and one-single BP measurement using an automated sphygmomanometer device. Logistic and linear regression models were used to estimate the odds of hypertension based on periodontitis case definitions. Mediation analysis was performed to understand the effect of age on the association of periodontitis with hypertension. Adjusted logistic model for gender, smoking habits and body mass index confirmed the association between high BP and periodontitis (OR = 2.31, 95%CI: 1.75–3.04, p < 0.001). Among 168 participants with undiagnosed high BP (15.9% of the study sample), 62.5% had periodontitis (n = 105). In this study, the association between periodontitis with both systolic blood pressure (SBP) (77.6%, p < 0.001) and diastolic blood pressure (DBP) (66.0%, p < 0.001) was mediated by age. Periodontitis is closely linked to BP in a representative Portuguese population.