This study was for the first time to investigate the effects of condensed tannins (CT) on intestinal immune function in on-growing grass carp (Ctenopharyngodon idella). A total of 540 healthy grass carp were fed six diets containing diﬀerent levels of CT (0, 10.00, 20.00, 30.00, 40.00 and 50.00 g/kg diet) for 70 days and then challenged with Aeromonas hydrophila for 14 days. The results showed that, compared with the control group, dietary CT (1) induced intestinal histopathological lesions and aggravated enteritis; (2) decreased the lysozyme and acid phosphatase activities, complement 3 (C3), C4 and immunoglobulin M contents, and down-regulated the Hepcidin, liver-expressed antimicrobial peptide(LEAP)-2A, LEAP-2B, Mucin2 and β-defensin-1 mRNA levels in the proximal intestine (PI), mid intestine (MI) and distal intestine (DI) (P < 0·05); (3) down-regulated the mRNA levels of anti-inflammatory cytokines transforming growth factor (TGF)-β1, TGF-β2 (not in MI and DI), IL-4/13A (not IL-4/13B), IL-10 and IL-11 partly correlated with target of rapamycin (TOR) signalling; (4) up-regulated the mRNA levels of pro-inflammatory cytokines interferon-γ2 (IFN-γ2), interleukin-1β (IL-1β), IL-6, IL-8 (not in PI), IL-12p35, IL-12p40, IL-15 and IL-17D partly related to nuclear factor kappa B (NF-κB) signalling in the intestine of on-growing grass carp. Overall, the results indicated that CT could impair the intestinal immune function, and its potential regulation mechanisms were partly associated with TOR and NF-κB signalling pathways. Finally, based on the PWG and enteritis morbidity, the maximum allowable levels of CT for on-growing grass carp (232.22-890.11g) were estimated to be 18.6 and 17.4 g/kg diet, respectively.