The widespread presence of mercury (Hg) and its derivatives in aquatic environments necessitates the investigation of their toxic effects on fish and the potential risks they pose to the health of local populations. The objective of this study was to investigate the toxic effects of mercury chloride (HgCl₂) on Oncorhynchus mykiss (Walbaum. 1792) by examining the dynamics of oxidative and nitrosative stress in muscle, kidney, and heart tissues. The rainbow trout used in the study had an average weight of 125.65±8.30 g. Five experimental groups were created, including a control group and groups exposed to 25% LC50 concentration of HgCl₂ for 7 and 14 days, as well as groups exposed to 50% LC50 concentration of HgCl₂ for 7 and 14 days. A total of 120 fish, with 40 fish assigned to each tissue (muscle, kidney, and heart), were used in the study. To evaluate oxidative/nitrosative stress, various parameters including ONOOˉ, TOS, TAS, OSI, and MDA levels were measured. Statistical analysis revealed a significant difference between the experimental groups in terms of TAS, TOS, OSI, ONOOˉ, and MDA levels in all three tissues (muscle, kidney, and heart) (P<0.05). In conclusion, the findings of this study demonstrate that HgCl₂ induces stress and toxicity in rainbow trout, as evidenced by increased levels of MDA and ONOOˉ, which are stress indicators in the fish.