Background: Interleukin-29 (IL-29) is one of the cytokines which has immunomodulatory properties and might play a role in the pathogenesis of periodontal diseases. The aim of this study was an immunohistochemical analysis of IL-29 in gingival tissues of chronic and aggressive periodontitis. Materials and Methods: In this cross-sectional study based on clinical evaluation and inclusion and exclusion criteria, 20 patients with generalized chronic periodontitis, 13 patients with generalized aggressive periodontitis, and 20 periodontally healthy individuals were enrolled. Gingival tissue samples were obtained during periodontal flap and crown lengthening surgery in periodontal patients and healthy individuals, respectively. Tissue samples were examined to determine the level of IL-29 expression by immunohistochemistry. The data were analyzed using SPSS and paired t-test, ANOVA test, and Tukey’s test (P < 0.05). Results: A total of 53 participants (34 females and 19 males) were enrolled in this study. IL-29 expression in the connective tissue of the patient groups was more than the healthy one (P < 0.001). In the aggressive periodontitis group, there was a significant increase of IL-29 expression compared to the other two groups, but there was no significant difference between the chronic periodontitis and healthy groups. Conclusion: According to the results of this study, IL-29 expression was increased in the gingival tissue of aggressive and chronic periodontitis. IL-29 local expression in aggressive periodontitis is higher than the chronic periodontitis and healthy groups, which could suggest the role of IL-29 in the etiopathogenesis of aggressive periodontitis.