OBJECTIVE: High-intensity laser therapy (HILT) is a relatively new form of Nd: YAG laser. The aim of the study is to investigate the additional benefits of HILT with conventional physiotherapy, related to pain and function, in patients with knee osteoarthritis (KOA). METHOD: The study comprised 43 knees from 31 patients of both genders with mean age 54.6 ± 6.22 (41-64) years. 53.49% of the knees were Kellgren Lawrence (KL) grade 2, and rest were KL grade 3 KOA. Group 1 (n = 21) received transcutaneous electrical nerve stimulation (TENS), hot packs (HP), exercises (EX), and HILT (Nd: yag-laser, 10 W). Group 2 (n = 22), received the same interventions but placebo HILT. All interventions were applied for 10 sessions. The Visual Analog Scale (VAS), Western Ontario & McMaster Universities Osteoarthritis Questionnaire (WOMAC), and Lequesne Algofunctional Index (LAI) were administered before, after, and at 12-week follow-up. RESULTS: Baseline VAS, WOMAC, and LAI scores of the groups were similar (p > 0.05). After treatment and 12 weeks of follow-up, both groups had significant relief for VAS, WOMAC, LAI pain (respectively, p < 0.001) and function (p < 0.012), except LAI-walking distance (p = 0.415). Post-hoc analyses and mixed-effects models showed no significant differences between groups over time for all variables. CONCLUSIONS: HILT did not provide additional short- or mid-term benefits in pain or function when added to a conventional physiotherapy and exercise program in patients with stage 2 or 3 knee osteoarthritis under 65 years of age.