Statement of Problem: Although mechanical debridement is considered as the conventional technique in the management of chronic periodontitis, the locally delivered antiseptic agents have also been investigated as an adjunctive therapy.Purpose: The purpose of this study was to investigate the clinical effectiveness of subgingival irrigation with polyvinylpyrrolidone-Iodine (PVP-I) 10%, H2O2 3%, and the combination of both in the measurement of probing depth and plaque and gingival indices of patients with moderate to severe chronic periodontitis.Materials and Method: In this double-blind randomized clinical trial, 16 patients with moderate to severe periodontitis were selected using the simple random sampling method. They had at least one tooth with a probing depth ≥5 mm in each quadrant and had undergone phase I of periodontal therapy one month after dental scaling, The initial probing depth, plaque, and gingival indices were recorded and the selected teeth were randomly irrigated with PVP-I 10%, H2O2 3%, H2O2 3% + PVP-I 10%, or normal saline. The measurements were repeated five weeks after the procedure. The data were analyzed through running paired-samples t-test, analysis of variance (ANOVA), Wilcoxon Signed Rank Test, and Kruskal-Wallis Test.Results: The mean differences in probing depth before and after subgingival irrigation in patients who were treated with normal saline, H2O2,PVP-I, and PVP + H2O2 were 1.29 mm, 1.35 mm, 1.47 mm, and 1.71 mm, respectively. This indi-cated a significant difference among all the groups ( p <05(. Furthermore, PVP-I had a positive effect on the gingival index but it had no significant effect on the plaque index.Conclusion: Subgingival irrigation is an effective adjunctive therapy to mechanical debridement in treating moderate to severe chronic periodontitis.