Codium fragile (CF) is a type of green algae consumed as kimchi in Asia. UPLC-QTOF-MS/MS analysis showed that CF contain lysophosphatidyl choline, canthaxanthin, retinoic acid, α-tocopherol, and unsaturated fatty acids, which reportedly improve skeletal muscle health. However, the effect of CF on skeletal muscle mass and function remains to be elucidated. In mice fed with CF extracts, exercise endurance and muscle weight increased. CF extracts enhanced protein synthesis and myogenic differentiation through the mTORC1 pathway. CF extracts also promoted oxidative muscle fiber formation and mitochondrial biogenesis through the PGC-1α-related signaling pathway. Upregulation of PGC-1α by CF extracts was abolished by EX527 SIRT1 inhibitor treatment. Changed signaling molecules in the CF extracts were partially regulated by canthaxanthin, a new compound in CF extracts, suggesting that canthaxanthin contribute synergistically to the effect of CF extracts. Therefore, CF is a potential food source for sport nutrition or prevention of sarcopenia.Copyright © 2021 Elsevier Ltd. All rights reserved.