Photobiomodulation therapy (PBMT) is a method currently used in the treatment of hard and soft tissue injuries due to its accelerating and enhancing effects on healing. In this study, we aimed to evaluate the possible additional benefits of applying PBMT with nonsurgical periodontal treatment in type 2 diabetes mellitus (DM) patients with chronic periodontitis (CP). Twenty-two type 2 DM patients with CP were enrolled in this clinical split-mouth study. Probing pocket depth (PPD), gingival index (GI), plaque index (PI), and clinical attachment level (CAL) were measured by intracaliber clinician (H.G.) at baseline and at 1 m, 3 m, and 6 m after treatment. Gingival crevicular fluid (GCF) samples were collected at baseline and at 1 week and 1 m, 3 m, and 6 m after treatment. According to split-mouth design, one randomly selected quadrant was treated with PBMT + nonsurgical periodontal treatment (NSPT) and the other quadrant was treated only non-surgical periodontal treatment. PBMT was applied the test quadrant on NSPT day and first, third, and seventh day after treatment at an energy density of 7.64 J/cm(2). Repeated measures analysis of variance test was used for the intragroup comparison and a "paired t test" in the intergroup comparison of the clinical and laboratory findings. Comparing the test and control quadrant after treatment, the test quadrant showed significant decrease in PPD at 1 month, 3 months, and 6 months; in GI at 3 months and 6 months; in CAL at month 6; in GCF at 1 week, 1 month, 3 months, and 6 months; and in IL-1 beta data at 3 months in comparison to the control quadrant. In contrast, there was no statistically significant difference in PI data at all times. Within the limitation of this study, adjunct use of PBMT on NSPT in patient with DM may positively affect the clinical and biochemical parameters.