The green mud crab has been widely cultured in coastal areas of southern China for many years, with production reaching approximately 14.1 tons in 2015. The successful farming of mud crabs presently depends on conventional diets, comprising mainly trash fish, mollusk meat and animal viscera, but the demand for these foods by the crab industry is difficult to satisfy. Compared with the traditional feeds, compound feeds offer better-balanced nutrition and have better storage qualities. Studies of the nutritional requirements of cultured mud crab are important to developing cost-effective, environmentally friendly and nutritionally balanced artificial diets. Dietary protein, a major macronutrient in aquatic animal diets, provides the essential and non-essential amino acids needed to synthesize body protein and energy. As protein represents the most expensive component in feeds, it is important to determine the optimal level for a cultured species. This study investigated the effects of dietary protein level on growth performance, body composition, and digestive enzyme activities in mud crab () juveniles. Six isoenergetic diets were formulated with varying protein content (i.e., 31.45%, 36.37%, 41.55%, 46.13%, 51.72% and 56.86%) and fed to juvenile mud crabs for three weeks. Triplicate groups of 80 juvenile crabs, with an average body weight of 11.86±0.11 mg, were randomly assigned to each of the six diets. Each crab was raised in a bucket (20 cm diameter × 25 cm height) provided with a 5 cm layer of sand. The results showed that the dietary protein level did not influence crab survival rate in any of the treatments (<0.05). The weight gain ratio and specific growth rate of the juvenile crabs significantly increased as dietary protein was increased from 31.45% to 51.72% (<0.05), and then the indicators showed a slight decrease with a further increase to 56.86% protein; the gains were highest for crabs fed 51.72% protein. Dietary protein level significantly affected the whole-body crude protein, lipid and ash contents (<0.05), but did not show any significant influence on the moisture content of the crabs (>0.05). Values for crude protein content of crabs raised on one of the four higher protein diets (41.55%-56.86%) were significantly higher than that for crabs fed a lower protein diet (31.45% and 36.37%) (<0.05), while the whole-body lipid content significantly decreased as the dietary protein content was increased (<0.05). The trend in whole-body ash content was similar to that observed for weight gain ratio, which significantly increased at first and then decreased as the dietary protein content was increased (<0.05). With an increasing level of dietary protein, the protease activity significantly increased (<0.05). The level of dietary protein also affected lipase activity in the crabs. Under the present experimental conditions, juvenile green mud crabs fed 41.55%-56.86% protein exhibited the best growth performance, and regression analysis showed that the optimal level was 49.03% protein. These results will be helpful for formulating pelleted diets for commercial mud crab culture. Future studies of protein metabolism mechanisms and the interactions of proteins with other nutritional components are recommended for this species.