This randomized controlled clinical trial evaluates the adjunctive effect of subgingival administration of Lactobacillus reuteri probiotics in diabetic patients with Stage III Grade B or higher periodontitis receiving non-surgical periodontal therapy. Forty periodontal sites were randomly assigned to either a test group treated with scaling and root planing (SRP) plus subgingival Lactobacillus reuteri or a control group treated with SRP plus saline irrigation. Clinical periodontal parameters, microbiological outcomes, and glycated hemoglobin (HbA1c) levels were assessed at baseline, 1 month, and 3 months. The study aims to determine whether locally delivered probiotics provide additional clinical, microbiological, and glycemic benefits compared with SRP plus saline irrigation.