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For aquatic animals, histidine is an essential amino acid. This study examined its impact on muscle growth and protein deposition in grass carp, both in vivoand in vitro, and explored the underlying mechanisms. A total of 450 grass carp (594.63 ± 0.68 g) were assigned randomly to six groups of three replicates for each group that was fed with six diets containing 1.08 (basal diet), 2.91, 5.87, 8.83, 11.78, and 14.79 g/kg histidine for 63 d. Results indicated that based on PWG and muscle protein content, the histidine demands of adult grass carp were 8.94 (34.12 g/kg protein) and 8.54 g/kg diet (32.60 g/kg protein), respectively. Optimal histidine promoted myofiber hypertrophy, which could be correlated with cell proliferation-associated proteins [PCNA, E2F4 and Cyclin (B, D and E)] and myogenic regulators (MyoD, MyoG, MyHC and Myf6). Optimum histidine enhanced protein deposition, which could be correlated with the facilitation of protein synthesis (IGF-1/PI3K/AKT/TOR) as well as the suppression of ubiquitin-proteasome system and autophagy-lysosome system to inhibit protein degradation. Furthermore, optimal histidine raised muscle bound amino acid content and amino acid chemistry scores, thereby increasing muscle nutritional value.