Knee osteoarthritis (KOA) is one of the most common musculoskeletal disorders causing pain and functional impairment. The purpose of the study is to compare the effects of high-intensity laser therapy (HILT), conventional physical therapy (CPT), and exercise therapy (ET) on pain and function in patients with KOA. The study was designed as an assessor-blind randomized controlled trial. Ninety-three patients (aged between 50 and 75 years) with proved KOA were included and randomly allocated into three groups, and received 12 sessions of HILT, CPT, or ET. The outcomes were pain intensity measured by visual analog scale (VAS), knee flexion range of motion (FROM), timed up and go test (TUG), 6-min walk test (6MWT), and functionality of knee measured by the Western Ontario and McMaster Universities Osteoarthritis (WOMAC) questionnaire. Statistical analyses were done to compare the amounts at the baseline, immediately after treatment and after 12 weeks. HILT was significantly more effective than the other groups in decreasing the VAS, increasing FROM and improving the scores of WOMAC (total and function subscale) both after treatment and after 12 weeks. The effect of HILT and CPT on the TUG, 6MWT, and WOMAC pain subscale was not significantly different after treatment, and both were better than ET. HILT was significantly better than the others after follow-up, particularly more effective on the stiffness subscale of WOMAC. HILT combined with exercise therapy, as a useful therapeutic approach, could have positive influences on KOA patients.