Obesity is a very prevalent chronic disease worldwide and has been suggested to increase susceptibility of periodontitis. The aim of this paper was to provide a systematic review of the association between obesity and periodontal disease, and to determine the possible mechanisms underlying in this relationship. A literature search was carried out in the databases PubMed-Medline and Embase. Controlled clinical trials and observational studies identifying periodontal and body composition parameters were selected. Each article was subjected to data extraction and quality assessment. A total of 284 articles were identified, of which 64 were preselected and 28 were finally included in the review. All the studies described an association between obesity and periodontal disease, except two articles that reported no such association. Obesity is characterized by a chronic subclinical inflammation that could exacerbate other chronic inflammatory disorders like as periodontitis. The association between obesity and periodontitis was consistent with a compelling pattern of increased risk of periodontitis in overweight or obese individuals. Although the underlying pathophysiological mechanism remains unclear, it has been pointed out that the development of insulin resistance as a consequence of a chronic inflammatory state and oxidative stress could be implicated in the association between obesity and periodontitis. Further prospective longitudinal studies are needed to define the magnitude of this association and to elucidate the causal biological mechanisms.