This study examined the correlation between temperature and the concentrations of lead, nickel, and cadmium heavy metals in the Tigris River water. It also attempted to assess the impact of these heavy metals on the histological changes in the gills of Chondrostoma regium. Three locations were chosen along the Tigris River in Baghdad city: one in the north, one in the middle, and one in the south. These locations were used to gather water and fish samples during a period of six months, from January to March and July to September 2023. A flameless atomic absorption spectrometer, specifically the Buck model, was employed to identify and quantify the presence of heavy metals in the river. Furthermore, a total of 180 Chondrostoma regium fish were examined histopathologically throughout the same time frame. It was employed to investigate the histopathological alteration in the gills. The study findings revealed the presence of lead, nickel, and cadmium heavy metals exceeding the maximum permissible limit. The elevation is primarily observed during the months with high temperatures (July, August, and September), in contrast to (January, February, and March). During the period from January to March, Region 3 exhibited a lowered concentration of lead, while Region 2 had lowered levels of nickel and cadmium. Conversely, Region 3 showed a higher concentration of cadmium, while Region 1 had higher levels of lead and nickel. From July to September, Region 2 had a lowered concentration of lead and nickel, while Region 1 had a lowered concentration of cadmium. Conversely, Region 3 had higher levels of lead, nickel, and cadmium. In addition, the impacts of increased levels of heavy metals on aquatic biota were assessed by examining the gills of Chondrostoma regium fish. This examination indicated an alteration in the structure of the secondary lamellae and the death of epithelial cells, accompanied by the secretion of mucus. The disparity was more pronounced during July, August, and September as opposed to January, February, and March. The study demonstrated an obvious connection between the concentration of heavy metals and the temperature of the Tigris River, which impacted the aquatic biota and had implications for public health. Moreover, the study's data provides insights into the potential biohazard impact of global climate change.