Objective : Knee osteoarthritis (KOA) and age are associated with high sarcopenia risk. This study aimed to investigate the effect of muscle strengthening exercise therapy (MSET) on muscle mass gain and muscle hypertrophy in older patients with knee osteoarthritis (KOA). Methods: A comprehensive search of online databases, namely PubMed, EMBASE, CINAHL, The Cochrane Library, the Physiotherapy Evidence Database (PEDro), the China Knowledge Resource Integrated Database, and Google Scholar databases, until March 2019 was performed in accordance with PRISMA guidelines. Randomized controlled trials (RCT) which reported the efficacy of MSET on lean mass gains, muscular morphology changes including muscle thickness and cross-sectional area (CSA) in elder people with KOA were identified. The included RCTs were analyzed through meta-analysis and risk of bias assessment. Results: We included 14 RCTs with a median PEDro score of 6/10 (range, 3/10 to 7/10). A total of 1,012 patients with a mean age of 61.6 years were analyzed. The MSET intervention resulted in a significantly higher gain in lean body mass with a standard mean difference (SMD) of 0.45 (95% confidence interval [Cl]=0.24, 0.66; P <0.0001). The results showed significant favorable effects of MSET on muscle thickness (SMD=0.64, 95% Cl=0.03, 1.24; P=0.04) and muscle CSA (SMD=0.54, 95% Cl=0.20, 0.87; P=0.002). Conclusion: MSET is effective in increasing lean body mass and muscle size in older adults with KOA. Clinicians should incorporate strategies for improving muscle function into their management of patients who have a risk of low muscle mass to maximize health status, especially for older individuals with KOA.