OBJECTIVE: This study aimed to identify the most frequently reported outcomes in sarcopenia trials, as a basis for developing a Core Outcome Set (COS). MATERIAL AND METHODS: A systematic review was conducted with a comprehensive search of MEDLINE, Embase, and Cochrane Central Register of Controlled Trials from database inception to March 2024 (PRISMA guidelines, PROSPERO: CRD42024525506). Eligible studies were randomized controlled trials (RCTs) on sarcopenia treatment, including patients diagnosed with a consensus definition. All outcomes reported in those studies were extracted and grouped into predefined subcategories. RESULTS: Among 3985 references, 58 studies (54 unique RCTs) met the inclusion criteria, reporting 214 efficacy and 39 safety outcomes. The most frequently reported outcomes were muscle mass (50 studies, i.e., present in 86.2% of studies), muscle strength (50 studies, 86.2%), physical performance (46 studies, 79.3%), nutrition-related outcomes (29 studies, 50.0%), and fat mass (28 studies, 48.3%). Among the primary outcomes, change in muscle mass was the most commonly reported outcome (23 studies, 39.7%, with appendicular skeletal muscle mass index and fat-free mass as the predominant measures), followed by change in muscle strength (16 studies, 27.6%, mainly assessed by handgrip strength), followed by change in physical performance (13 studies, 22.4%, mainly assessed by gait speed and lower limb physical function). Secondary outcomes included biomarkers, physical status, quality of life, activities of daily living, bone mass, psychological status, cognitive function, and unclassified outcomes. CONCLUSIONS: This review highlights the extensive heterogeneity of reported outcomes in sarcopenia intervention studies and the urgent need for developing a COS.