To assess the role of dietary creatine on myofiber characteristics and protein synthesis in muscle, we fed grass carp (Ctenopharyngodon idellus, initial body weight: 88.47 ± 1.44 g) creatine-supplemented diets (1.84, 5.91, 8.48, and 15.44 g/kg diet) for 8 weeks. Creatine supplementation did not affect growth performance, but significantly increased creatine contents in muscle and liver. At 8.48 g/kg, creatine decreased the activities of alanine transaminase and aspartate aminotransferase in serum, and improved hardness and chewiness of muscle due to shorter myofiber mean diameter, higher myofiber density and the frequencies of the diameters of class I and III and collagen content, longer sarcomere length, and upregulated mRNA levels of slow myosin heavy chains. Creatine supplementation upregulated the mRNA expressions of myogenic regulatory factors. The 8.48 g/kg creatine-supplemented diet significantly increased the contents of protein, total amino acids (AAs), essential AAs, and free flavor AAs in muscle, the protein levels of insulin-like growth factor I, myogenic differentiation antigen, and peroxisome proliferator-activated receptor-γ coactlvator-1α in muscle, and stimulated the phosphorylation of target of rapamycin (TOR) pathway in muscle. In summary, 8.48 mg/kg creatine improved fish health and skeletal muscle growth, and increased hardness and protein synthesis in muscle of grass carp by affecting myofiber characteristics and the TOR signaling pathway. A second-order regression model revealed that the optimal dietary creatine supplementation of grass carp ranges between 8.48 and 12.04 g/kg.