The aim of this systematic review was to evaluate the actual efficacy ofLactobacillus reuteri(L. reuteri) on the periodontal clinical parameters when used concomitantly to the nonsurgical periodontal treatment. Searches were conducted through PubMed Central, Online Knowledge Library, Science Direct, Scielo, and Cochrane databases from 2012 to 2022. The focused question was “In patients with periodontitis, will the probioticL. reuteri, when administrated as an adjunct to nonsurgical periodontal treatment, compared to the nonsurgical periodontal treatment alone, result in better clinical outcomes?” The following information was extracted from the articles: author and year of publication, type of study, follow-up, sample size and number of defects, and clinical characteristics and details. All included studies were qualitatively assessed using the Critical Appraisal tools according to the Joanna Briggs Institute. Twenty-four articles were full-text reading, but only 9 articles were included. The number of patients enrolled was 287, aged between 18 and 56 years. All periodontal parameters were evaluated. The “follow-up” varied (14, 40, 84, 90, 180, and 360 days). Most articles supported the clinical benefits ofL. reuterias an adjunct to SRP compared to SRP alone. A common finding at the beginning period was thatno statistically different results were observed between the test and control groups; otherwise, at the last period, a significant improvement was found in favor of the probiotic use (p= 0.001) for all the clinical parameters. The use ofL. reuterias an adjunct to nonsurgical periodontal treatment may result in significantly better clinical outcomes than nonsurgical periodontal treatment alone; but the conclusion must be carefully interpreted because of the heterogeneity found among the studies.