Background: Periodontitis is a prevalent chronic inflammatory disease marked by progressive periodontal tissue destruction. It is classified according to the 2017 AAP/EFP staging and grading framework and managed in accordance with the EFP S3-level guideline. Professional mechanical plaque removal (PMPR) remains the cornerstone of non-surgical therapy. However, its limited effectiveness in deep periodontal pockets has prompted interest in adjunctive probiotic approaches. These include Lactobacillus plantarum and Lactobacillus brevis, which exhibit antimicrobial and immunomodulatory properties. Methods: A PRISMA 2020–compliant systematic search of five databases identified randomized controlled trials evaluating L. plantarum and L. brevis as adjuncts to PMPR in periodontitis. The findings were synthesized descriptively. Results: Adjunctive subgingival delivery of a probiotic gel containing Lactobacillus plantarum and Lactobacillus brevis was associated with significant improvements in PPD, CAL, and IL-10 compared with PMPR alone. In contrast, systemic or combined application showed inconsistent or nonsignificant effects. Clinical benefits were more evident in stage I–II periodontitis, whereas evidence in stage III–IV disease remained inconclusive. Discussion: Three randomized controlled trials were included. While clinical parameters improved following PMPR in all groups, additional benefits from probiotics were observed primarily in early-stage periodontitis with subgingival delivery. Findings in advanced-stage disease were inconsistent and often not statistically significant, suggesting that treatment response may depend on disease stage and delivery method. Conclusion: Adjunctive subgingival application of Lactobacillus plantarum and Lactobacillus brevis may offer short-term clinical benefits, particularly in stage I–II periodontitis. However, evidence in stage III–IV remains inconsistent and often not statistically significant. Given the limited number of heterogeneous trials, further well-designed studies are needed to establish their clinical relevance. © The Author(s) 2026.