In this study, the effects of dietary astaxanthin (Lucantin Pink®) at 0.5 and 2 g kg−1 incorporated in the control diet were investigated on growth performance, biochemical parameters, including protein and lipid oxidation levels, peroxide value, pH level and water content as well as gene expression level related to catalase, glutathione peroxidase, glutathione S‐transferase and superoxide dismutase in rainbow trout (Oncorhynchus mykiss) fillet. The fish (mean weight 20.69 g) were randomly distributed among 12 tanks with 10 fish per tank and fed different diets for 60 days. On day 60, each group was divided into two subgroups. In one of the subgroups, fish were exposed to paraquat (PQ) challenge at a level of 4.82 mg/L for 96 h. The other subset was used as the negative control. The results indicated that the treated diets enhanced the growth performance compared with the control diet. The protein and lipid oxidation levels and peroxide value also decreased significantly with the enriched diets compared with the control group exposed to PQ. Conversely, no significant changes were noted in the pH level and water content following the challenge with PQ. Feeding fish astaxanthin and challenging them with PQ also up‐regulated the expression of the mentioned antioxidant‐relevant genes in rainbow trout fillet. Based on the results, it was concluded that commercial astaxanthin improved the growth performance and attenuated the oxidative stress induced by PQ in rainbow trout fillets by enhancing the functions of the antioxidant system.