This study was carried out to understand the preventive effect of selenium (Se4+) on heavy metal stress induced by lead and copper in rainbow trout (Oncorhynchus mykiss). Variation in glutathione peroxidase (Se-GSH-Px) and superoxide dismutase (SOD) activity, and in malondialdehyde (MDA) levels in liver, spleen, heart, and brain tissues of rainbow trout after 72 h of exposure to Pb2+ and Cu2+ were investigated in the presence and absence of Se4+. In the presence of Se4+, Se-GSH-Px activity and SOD activity were found to be higher and MDA levels were lower compared with in its absence. Hematological parameters were also determined and it has been observed that total leukocyte count (WBC), mean cell volume (MCV), and mean cell hemoglobin (MCH) were increased and erythrocyte number (RBC), hemoglobin (Hb), and hematocrit value (Hct; P < 0.05) were decreased in fish exposed to heavy metals in the absence of selenium. Selenium presence recovered hematological parameters to normal levels. In the light of our findings, it could be stated that Pb2+ and Cu2+ lead to dramatic changes in biochemical and hematological parameters and selenium caused these parameters to converge to control levels when it was administered concurrently with these heavy metals.