IntroductionKnee osteoarthritis (KOA) is a highly prevalent disabling joint disease. Intra-articular stem cell therapy is increasingly being used for treating KOA with little high-quality evidence to support its use. The aim of this study is to investigate the efficacy, safety and cost-effectiveness of allogeneic mesenchymal stem cells (Cymerus MSCs) for treating symptomatic tibiofemoral KOA and improving knee structure over 24 months.Methods and analysisThe Stem Cell injections for symptomatic relief and strUctural improvement in people with Tibiofemoral knee OsteoaRthritis study is a phase III, multi-centre, parallel, superiority, randomised, double-blind, placebo-controlled trial, which will be conducted in Sydney and Hobart, Australia. 440 participants (220 per arm) aged over 40 years with painful KOA and mild to moderate structural change on X-ray (Kellgren and Lawrence grade 2 or 3) with medial minimum joint space width between 1 and 4 mm in the study knee will be recruited from the community and randomly allocated to receive either intra-articular MSCs or saline at baseline, week 3 and week 52. The coprimary outcomes will be the proportion of participants achieving patient-acceptable symptom state for knee pain at 24 months and quantitative central medial femorotibial compartment cartilage thickness change from baseline to 24 months. Main secondary outcomes include change in knee pain, Patient Global Assessment, physical function, quality of life and other structural changes. Additional data for cost-effectiveness analysis will also be recorded. Adverse events will be monitored throughout the study. The primary analysis will be conducted using modified intention-to-treat.Ethics and disseminationThis protocol has been approved by The University of Sydney (USYD) Human Research Ethics Committee (HREC) #: 2020/119 and The University of Tasmania (UTAS) HREC #: H0021868. All participants will be required to provide informed consent. Dissemination will occur through conferences, social media, and scientific publications.Trial registration numbersAustralian New Zealand Clinical Trials Registry (ACTRN12620000870954); U1111-1234-4897.