Purpose: Periodontitis is an infectious disease caused predominantly by gram-negative anerobes. The host inflammatory response to these bacteria causes alveolar bone loss that is characterized as periodontitis. Omega-3 fatty acids (${omega}$-3 FAs) have anti-inflammatory properties, thus have been used to treat some chronic inflammatory diseases such as cardiovascular disease and rheumatoid arthritis. We aimed to evaluate the effect of dietary supplementation with ${omega}$-3 FAs as a host modulating agent in patients with chronic periodontitis. Methods: Sixty otherwise healthy subjects with moderate and severe chronic periodontitis were enrolled in our randomised, double-blind, placebo-controlled trial. The control group (CG, n=30) was treated with scaling and root planing (SRP) and given a placebo; the treatment group (TG, n=30) was treated with SRP and dietary supplementation of ${omega}$-3 FAs (one 300 mg tablet daily for 12 weeks). Periodontal clinical parameters and serum C-reactive protein (CRP) levels were evaluated in all patients at baseline, a 6-week and 12-week period after treatment. Results: A significant reduction in the gingival index, sulcus bleeding index, pocket depth, and clinical attachment level was found in the TG compared to the CG at a 12-week period. However, no statistically significant changes in serum CRP levels were found. Conclusions: Our findings suggest that ${omega}$-3 FAs can successfully reduce gingival inflammation, pocket depth, and attachment level gain. Dietary supplementation with ${omega}$-3 FAs may have potential benefits as a host modulatory agent in the prevention and/or C management of chronic periodontitis.