Abstract: The mud crab (Scylla paramamosain) is the most farmed marine crab species in China. However, the lack of systematic research on its nutritional requirements has hindered the development of high-quality complete formulated feeds, which poses a major bottleneck to achieving sustainable, efficient, and green development of the industry. This study aimed to determine the optimal dietary leucine requirement for juvenile mud crabs S. paramamosain initial weight of (15.07±0.97) mg. Five isonitrogenous and isoenergetic diets were formulated with leucine levels of 2.09% (Leu 1), 2.39% (Leu 2), 2.66% (Leu 3), 2.99% (Leu 4), and 3.34% (Leu 5). After an 8-week feeding trial, crabs fed diets with leucine levels between 2.09% and 2.66% exhibited significantly improved growth performance, including final body weight (FBW), weight gain (WG), and specific growth rate (SGR). However, growth was significantly suppressed when leucine levels exceeded 2.99%. At the physiological level, appropriate leucine supplementation (2.09%-2.66%) enhanced digestive enzyme (trypsin (TRY)) and metabolic enzyme (alanine aminotransferase (ALT) and aspartate aminotransferase (AST)) activities in the hepatopancreas. Concurrently, the expression of key genes in the TOR signaling pathway (tor, akt, s6k1, eif4e2, 4e-bp21) was significantly upregulated. In contrast, excessive leucine (2.99%-3.34%) induced oxidative stress, as evidenced by suppressed antioxidant enzymes (superoxide dismutase (SOD) and catalase (CAT)) activities and elevated malondialdehyde (MDA) content. Immune-related gene expressions (lysozyme (lzm) and prophenoloxidase (proPO)) were also inhibited. Analysis of the hepatopancreatic microbiota revealed that optimal leucine levels promoted a higher abundance of Thiothrix, which was positively correlated with growth indicators, digestive and metabolic enzyme activity, and antioxidant capacity. Conversely, high leucine levels significantly increased the proportion of potential pathogens like Vibrio and Photobacterium, which were associated with oxidative damage. Based on a second-order polynomial regression model of WG, the optimal dietary leucine requirement for juvenile S. paramamosain was determined to be 2.41%. This study addresses the gap in key nutrient requirements data for juvenile S. paramamosain, laying an important theoretical foundation for the development of high-quality formulated diet.