Abstract β-glucans are currently a notable topic in research on enhancing immunity in fishes and their significant role in improving fish health and productivity in aquaculture. Phagocytic activity (PA), phagocytic index (PI), analyses of oxidative stress biomarkers (2-thiobarbituric acid reactive substances [TBARS] as biomarkers of lipid peroxidation, aldehyde and ketone derivatives of oxidatively modified proteins [OMP]), total antioxidant status (TAS), and lysosomal enzyme activity (alanyl aminopeptidase [AAP], leucyl aminopeptidase [LAP], acid phosphatase [AcP], and β-N-acetylglucosaminidase [NAG]) were used to assess the effects of dietary β-glucans on the skeletal muscle and heart tissues of European whitefish ( Coregonus lavaretus L.). Our study showed that dietary β-glucan supplementation improved the immune system of European whitefish. Fifteen, 30, and 45 days after supplementing diets with β-glucans, the fish showed increased blood PA and PI, indicating improved immune response. β-Glucans also reduced lipid peroxidation and oxidatively modified protein levels and effectively managed the overall antioxidant status. We observed an increase in β-N-acetylglucosaminidase (NAG) activity in the skeletal muscle tissue on days 15 and 30 after β-glucan dietary supplementation and in the cardiac tissue on days 15 and 45 after β-glucan dietary supplementation. This highlights the role of NAG in the metabolic changes in muscle caused by the action of β-glucans. Furthermore, NAG activity was significantly associated with the tissue type and the duration of β-glucan dietary supplementation, as shown by regression analysis. MANOVA analysis confirmed that the long-term effect of β-glucans was more pronounced. β-glucan supplementation is an effective strategy for improving immune function and managing oxidative stress in the skeletal and cardiac muscles of European whitefish.