Insecticides that disrupt the healthy functioning of the ecosystem have toxic potential on non-target organisms. Fish, an important component of the aquatic ecosystem, are exposed to these pesticides in different ways. The stress response is regarded as an adaptive mechanism that allows the fish to cope with the perceived stressor to maintain its normal or homeostatic state. This mechanism is determined by antioxidant parameters and oxidative stress indicators measured in gill and liver tissue of rainbow trout. Accordingly, the effects of fipronil (FP) insecticide on the physiology of rainbow trout (Oncorhynchus mykiss) were determined with using the different biomarkers (superoxide dismutase (SOD), catalase (CAT), glutathione peroxidase (GPX), malondialdehyde (MDA), paraoxonase (PON), arylesterase (ARE), myeloperoxidase (MPO), 8-hydroxy-2-deoxyguanosine (8-OHdG)), and caspase 3 activity) in this study. Different doses of FP inhibited antioxidant enzyme activities in rainbow trout liver and gill tissues while inducing oxidative stress parameter (MDA, MPO, and 8-OHdG) levels. Also, caspase-3 activity was increased in liver and gill tissue, but this increase was statistically significant only in gill tissue (p < 0.05). When the results of the study were taken into consideration, it was concluded that different doses of FP insecticide caused physiological changes in rainbow trout and the studied parameters were usable biomarkers in explaining the adaptive response of stress factor.