Chronic pain is a global public health problem, which intensifies even more in the obese population, reaching about 33% of these patients. Among the topography, chronic knee pain affects 80%, constituting an important cause of disability and decreased quality of life. In addition, in grade 3 obesity, also called morbid obesity, in which bariatric surgery is already indicated, knee pain makes it difficult or prevents participation in the pre-habilitation program that includes physical therapy through therapeutic exercises with the aim of reducing postoperative complications. In this sense, a double-blind, randomized, placebo-controlled clinical trial will be conducted with the objective of evaluating the effect of photobiomodulation (PBM) on pain and functionality of obese patients with chronic knee pain who are in a prehabilitation program for bariatric surgery, discussing its role as an analgesic therapy and modifier of peripheral and central sensitization mechanisms of the pain pathway. PBM is a safe treatment option, with no undesirable effects and low cost. The primary outcomes will be pain intensity, through visual analogue scale, and Knee injury and Osteoarthritis Outcome Score (KOOS) questionnaire, validated version for Portuguese - Brazil. Secondary outcomes will be pressure pain threshold and rolling pinch maneuver measured by digital algometer. Other exploratory outcomes will be 6-minute walk test, knee range of motion, SF-36 quality of life questionnaire. There will be 2 groups: an intervention group (photobiomodulation associated with standard physiotherapy treatment) and another placebo group (device turned off associated with the same standard physiotherapy treatment). The application sites will be knees and lumbar paravertebral 2 times a week for 12 weeks. The dosimetric standards will be 4J/point in the knees and 3J/point in the lumbar. The results obtained will be statistically analyzed and later published in a scientific journal.