<jats:p><jats:bold>Background:</jats:bold> Several recent studies have indicated that total antioxidant capacity (TAOC) of plasma seems to be compromised in chronic periodontitis (ChP). The aim of this study is to investigate plasma TAOC in patients with ChP and to assess the effects of vitamin C as an adjunct to non‐surgical periodontal treatment.</jats:p><jats:p><jats:bold>Methods:</jats:bold> The study population consisted of 60 subjects: 30 diagnosed with ChP and 30 matched controls. Furthermore, patients from the ChP group were randomly allocated into ChP1 (15 patients received non‐surgical treatment with adjunctive dose of vitamin C) and ChP2 (15 patients received non‐surgical periodontal treatment alone). Plasma TAOC levels were measured by an ABTS assay at baseline and 1 month post‐therapy.</jats:p><jats:p><jats:bold>Results:</jats:bold> Plasma TAOC levels were significantly lower in ChP patients than controls (<jats:italic>P</jats:italic> <0.001). The periodontal therapy resulted in increasing plasma TAOC and improvements in clinical measures among both ChP1 and ChP2 groups (<jats:italic>P</jats:italic> <0.001). However, the adjunctive dose of vitamin C did not offer additional effect (<jats:italic>P</jats:italic> >0.05).</jats:p><jats:p><jats:bold>Conclusions:</jats:bold> ChP is significantly associated with lower levels of plasma TAOC. The non‐surgical periodontal therapy seems to reduce the oxidative stress during the periodontal inflammation. However, the use of adjunctive vitamin C still needs further investigation.</jats:p>