This study aims to increase the width and thickness of peri-implant keratinized mucosa (KM) by transferring a properly dimensioned free gingival graft (FGG) from the palatal donor site and to evaluate its effect as an adjunctive approach to non-surgical peri-implantitis therapy. The procedure is indicated for patients diagnosed with peri-implantitis who present with an inadequate KM (midbuccal KM height \< 2 mm). One month after non-surgical therapy, patients in the test group will undergo keratinized mucosa augmentation using an FGG, while patients in the control group will continue with supportive periodontal therapy and regular follow-up visits. The primary outcome measure of the study is the change in probing depth (PD) at 12 months following peri-implantitis treatment.