BACKGROUND: Sarcopenia and physical frailty are age-related syndromes characterized by progressive loss of muscle mass and function, significantly impacting mortality and quality of life in older adults. This systematic review evaluated the effectiveness of protein supplementation interventions for these conditions. METHODS: We systematically searched Medline, CENTRAL, and Ichushi Web from January 2000 to March 2023, with additional manual searching extended to March 2024. Randomized controlled trials investigating protein supplementation, alone or combined with exercise, in adults aged ≥65 years with sarcopenia or physical frailty were included. The primary outcomes were changes in muscle mass, strength, and physical performance. RESULTS: The systematic literature search identified 1,506 records through database searching (Medline: 357, CENTRAL: 275, Ichushi Web: 639) and 235 additional records through hand searching. Finally, 13 randomized controlled trials (n=1,057) met the inclusion criteria. Combined protein and exercise interventions demonstrated significant improvements in skeletal muscle index (MD = 0.89 kg/m², 95 % CI: 0.45 to 1.33) and handgrip strength (MD: +2.64 kg, 95 % CI: +0.75 to +4.53) compared to exercise alone. Protein supplementation alone showed modest benefits in muscle strength but limited effects on physical performance. No serious adverse events were reported. CONCLUSIONS: While protein supplementation combined with exercise shows promising effects on muscle mass and strength in older adults with sarcopenia or physical frailty, the evidence quality was consistently rated as very low. Further high-quality trials are needed to establish optimal supplementation strategies. REGISTRATION: PROSPERO: CRD42023408529.