The present analysis aimed to assess the immunomodulation effects of β‐glucan (βG) against the sub‐lethal concentrations of herbicide pendimethalin (PMN). 360 Oreochromis niloticus fingerlings were randomly classed into four groups. The control one (CON) was fed on basal diet. βG group was fed on the basal diet accompanied with 0.5 βG g/kg−1. The PMN group was endangered to PMN at a dosage of 0.52 mg/L. βG/PMN group was supplemented with combination of βG and exposed to PMN as beforehand stated for eight weeks. Stunting of the growth parameters followed PMN was improved by βG. PMN revealed a significant decline in total number of leucocytes. Furthermore, reduced in immunoglobulin M (IgM), lysozymes, superoxide dismutase (SOD) and catalase (CAT), likewise, increased in malondialdehyde (MDA), cortisol, hepatic and renal markers by exposure to PMN. Also, PMN revealed up‐regulation of mRNA levels of IgM in the spleen. Conversely, the expression of CAT, SOD and il8 was down‐regulated. PMN increased the sensibility to Aeromonas hydrophila challenge, manifested by a significantly increased of cumulative mortality post challenge. The mortalities have been stopped at the 7, 8, 11 and 12th days in, βG, CON, βG/PMN and PMN respectively. Our results accomplish that PMN influences immune responses and antioxidant balance by the adverse transcriptional impact on genes concerned in immunity and also inflammatory indicators on immune organs. In addition, supplementation with βG could be beneficial for alleviates of the immunity in response to PMN exposure.