Inclusion Criteria:
* Clinically confirmed knee osteoarthritis (OA) based on the American College of Rheumatology (ACR) criteria with symptomatic pain for ≥ 3 months
* Average knee-pain intensity ≥ 30/100 on both the Daily Symptom Diary (DSD) run-in and CATI
* Age ≥ 50 years
* Ability to give informed consent, attend visits, and complete ≥ 4 of 7 DSD entries pre-randomization
* Capacity to perform all study tasks (Timed Up-and-Go, knee OA assessments, blood draw, questionnaires, etc)
Exclusion Criteria:
Systemic and Rheumatic Disease
* Active systemic rheumatic condition (e.g., rheumatoid arthritis, systemic lupus erythematosus)
* Fibromyalgia with pain outside the knee that is equal to or worse than knee pain
Musculoskeletal History
* Clinically significant surgery on the index knee (e.g., arthroplasty, osteotomy)
* Knee surgery within the past 6 weeks
* Intra-articular injection to the index knee within the past 14 days
Medication Use
* Daily opioid therapy
* Hypersensitivity to PBM
* Migraine triggered by light
* Open wounds, skin infections, eczema, burns, or other dermatologic conditions at or near the treatment application site.
* New prescription or OTC analgesic, non-pharmacologic pain therapy, chemotherapy, or radiation therapy initiated ≤ 30 days before screening
Cardiovascular Safety
* Uncontrolled hypertension (SBP \> 150 mmHg or DBP \> 95 mmHg)
* Unstable or activity-limiting cardiovascular or peripheral arterial disease
Neurological and Psychiatric Conditions
* Diagnosed neurological disorder (e.g., Parkinson's disease, multiple sclerosis, epilepsy)
* History or evidence of stroke or moderate-severe traumatic brain injury
* Serious psychiatric illness requiring hospitalization within the past 12 months
* Active suicidal ideation
* Current substance-use disorder or past hospitalization for substance-use treatment
Dermatologic Conditions
* Compromised skin integrity at or near the treatment site (e.g., open wounds, active rash, dermatitis, burns, ulcers, eczema, infection) interfering with topical treatment or PBM
Other Metabolic Conditions
* Uncontrolled diabetes
* Diabetes-related complications (e.g., peripheral neuropathy, severe nephropathy, chronic ulcers at treatment site)
Other Pain Conditions
* Chronic pain at another body site more severe than knee pain
General Health and Cognition
* Pregnancy or lactation
* Cognitive impairment that precludes informed consent or task participation
