Objective: Pharmacological treatments of osteoarthritis (OA) have several side effects. Boswellia serrata resin (frankincense) is rich in boswellic acids that have antioxidant and anti-inflammatory effects; though, their oral bioavailability is low. The aim of this study was evaluation of the clinical effectiveness of frankincense extract in the treatment of knee OA. In a randomized double-blind placebo-controlled clinical trial, eligible patients with knee OA were randomly divided into two groups of drug (33 patients) and control (37 patients), to use oily solution of frankincense extract or placebo, respectively, on the involved knee three times daily for four weeks. WOMAC (Western Ontario and McMaster Universities Osteoarthritis Index), VAS (visual analogue scale; for pain severity), and PGA (patient global assessment) scores were determined before and after intervention. Results: For all evaluated outcome variables, there was a significant decrease from baseline in both groups (P < 0.001 for all). Furthermore, the end-of-intervention values for all parameters were significantly lower in drug group than placebo group (P < 0.001 for all), showing more effectiveness of drug compared to placebo. Conclusion: Topical oily solution containing enriched extract of boswellic acids could decrease pain severity and improve the function in patients with knee OA. Trial Registration Trial registration number: IRCT20150721023282N14. Trial registration date: September 20, 2020. The study was retrospectively registered in Iranian Registry of Clinical Trials (IRCT). [ABSTRACT FROM AUTHOR]