Background: The aim of this study was to evaluate the effect of photobiomodulation in a form of low-level laser therapy (LLLT) as an adjunct to conventional periodontal therapy in patients with moderate to advanced periodontitis. Methods: All 50 systemically healthy patients who were included in the study initially received conventional periodontal therapy. Both groups received the treatment at baseline, included scaling and root planing (SRP) using ultra-sonic scalers and hand instruments by the same operator under local anesthesia to minimize patient discomfort for the entire dentition in a single visit. The laser group (n = 25) received LLLT as an adjunct to the conventional periodontal therapy. The LLLT was of intensity 2.5 J/cm2 and a wavelength 980 nanometer. Three laser sessions were given at the first and second weeks, followed by two sessions during the third week, and then sessions were applied once weekly during the fourth and fifth weeks (a total of 10 sessions for every site). Plaque index (PI), gingival index (GI), pocket depth (PD), clinical attachment level (CAL) and visual analogue scale (VAS) were all measured at the baseline and after treatment. Data were collected and statistically analyzed.