Osteoarthritis (OA) is the leading cause of disability worldwide and knee is the commonest joint involved. In 2020 there were around 654.1 million individuals (40 years and older) with knee OA and this prevalence tend to increase rapidly due to the growing aging of the population as well as an increase in risk factors particularly obesity and sedentary lifestyle. Evidences show that OA is associated with chronic low grade inflammation. Nonsteroidal anti-inflammatory drugs (NSAIDs) are the mainstay of management for OA symptoms which however cause serious gastrointestinal, renal and cardiovascular adverse events. With the lack of safe and effective therapies for OA, many patients are eventually faced with costly total joint replacement as the only option to improve pain, function, and quality of life. Astaxanthin is a natural oxycarotenoid which exhibits strong anti-inflammatory and anti-oxidant effects and is a promising therapeutic potential in OA. Till date sufficient data is not available regarding effects of astaxanthin in OA in human. This study is an effort to explore whether astaxanthin as add on therapy with conventional treatment of OA produce improvement in disease activity and inflammatory indicators in moderate to severe knee OA patients. This study will be a randomized, double-blind, placebo-controlled trial and will be conducted in the Department of Pharmacology, BSMMU in collaboration with the Department of Physical Medicine and Rehabilitations, BSMMU. A total of eighty (80) OA patients attended in the outpatient Department of Physical Medicine and Rehabilitations, BSMMU will be selected for the study according to inclusion and exclusion criteria. Diagnosis of moderate and severe knee OA will be made radiologically by an expert physiatrist of Department of Physical Medicine and Rehabilitations, BSMMU using the Kellgren-Lawrence grading scale. Each participant will be assessed by translated and validated Bangla version of Western Ontario and McMaster Universities Osteoarthritis Index (WOMAC) VA3.01 in a calm place in the BSMMU campus for pain, stiffness and physical function. Serum Interleukin-6 (IL-6) and high sensitive C-reactive protein (hsCRP) will be measured. Then participants will randomly be assigned into two groups: Intervention group and Control group. The intervention group patients will receive conventional treatment with astaxanthin 3 capsules of 4mg after breakfast daily for 8 weeks making a total dose of 12 mg astaxanthin/day. On the other hand, patients in control group will receive 3 placebo capsules of 4 mg after breakfast daily for 8 weeks along with conventional treatment given by the physiatrist. The regularity of medicine intake will be ensured over telephone, capsule count and from the patient's compliance sheet. After 8 weeks of therapy each participant will be assessed once again by Bangla version of WOMAC Index and serum IL-6 and hsCRP will be measured. Patients will be asked to inform if they encountered any astaxanthin related adverse events. Study outcomes will provide evidence to support astaxanthin's safety and efficacy as an added strategy for symptom management in patients with knee osteoarthritis.