Background: Periodontitis and type 2 diabetes (T2D) maintain a bidirectional pathophysiological relationship that impairs conventional periodontal treatment outcomes. Chitosan, a biopolymer derived from chitin, possesses well-documented antimicrobial, hemostatic, wound-healing, and osteo-inductive properties. Objective: We aimed to evaluate whether 1% chitosan gel delivered nightly via a custom occlusal guard, as an adjunct to full-mouth scaling and root planning (SRP), would improve periodontal clinical and radiographic outcomes in patients with T2DM and Stage I-II periodontitis compared to placebo. Methods: A double-blind, randomized, placebo-controlled, parallel-group clinical trial was conducted (n = 16; 8 per group) in adults aged 40-65 years with T2D and periodontitis. Following SRP, participants received either 1% chitosan gel or placebo nightly via a custom occlusal guard for 30 days. Primary outcomes included periodontal pocket depth (PPD), clinical attachment level (CAL), and radiographic bone level (RBL). Secondary outcomes were probing depth (PD), bleeding on probing (BOP), and Miller mobility grade. Statistical analysis employed the Wilcoxon signed-rank test (within-group) and Mann-Whitney U test (between-group); p < 0.05 was considered significant. Results: Groups were comparable at baseline. Within-group analysis at 30 days revealed significant reductions in PPD (4.2 +/- 1.1 to 3.4 +/- 1.1 mm; p = 0.019) and RBL (5.6 +/- 2.2 to 5.1 +/- 1.9 mm; p = 0.030) in the chitosan group, and a clinically meaningful trend in CAL (5.1 +/- 2.8 to 3.8 +/- 1.7 mm; p = 0.069). The placebo group showed significant BOP reduction (78.3 +/- 23.4 to 51.5 +/- 26.0%; p = 0.017), consistent with the known effect of mechanical debridement. No between-group differences reached significance in delta values. Conclusions: Adjunctive use of 1% chitosan gel via a custom occlusal guard as a delivery system produces clinically relevant improvements in PPD and RBL in T2D patients with periodontitis. The occlusal guard represents a novel, patient-administered, sustained-contact delivery platform for periodontal local therapy. Larger, long-term randomized trials with microbiological and glycemic endpoints are warranted.