The role of leucine (Leu) in the regulation of the intestinal immune status, immune-related signalling molecules and tight junction (TJ) protein transcript abundance in the intestine of grass carp (Ctenopharyngodon idella) was investigated. Six iso-nitrogenous diets that contained graded levels of Leu (7.1–17.1�g�Leu�kg^−�1 diets) were fed to the fish for 8�weeks. Compared with the control group, appropriate Leu supplementation increased (P�<�0.05) the following: (1) the lysozyme activity, acid phosphatase activity and complement component 3 (C3) content in all intestinal segments; (2) the mRNA levels of interleukin 10, inhibitor factor κBα (IκBα) and target of rapamycin (TOR) in the mid and distal intestine as well as transforming growth factor β1 in all intestinal segments; and (3) the transcript levels for claudin b, claudin 3, claudin 15, occludin and zonula occludens-1 (ZO-1) in the intestine of young grass carp. At the same time, appropriate Leu supplementation down-regulated the mRNA levels of tumour necrosis factor α, interleukin 8 and nuclear factor κB p65 (NF-κB p65) in the mid and distal intestine of young grass carp (P�<�0.05). Interestingly, the transcript levels for claudin c and claudin 12 showed no significant differences among the groups in the intestine of young grass carp. In conclusion, the positive effect of Leu on intestinal health is associated with the improvement of the intestinal immune status and the regulation of immune-related signalling molecules and tight junction transcripts of fish.