Abstract: Opsariichthys bidens, a commercially important freshwater fish widely distributed in rivers and lakes across East Asia, is known for its delicious meat and abundant nutrients. Mercury is a common heavy metal pollutant in aquatic environments, adversely affecting fish behavior, growth, immunity, and reproductive systems. In contrast, Spirulina platensis is rich in protein, has a balanced amino acid composition, and is easily digested and absorbed, making it highly effective in promoting the healthy growth of aquatic animals. This study evaluated the effects of varying concentrations of S. platensis on the growth performance, antioxidant indicators, immune function, and heat shock proteins (HSP) expression in O. bidens under mercury stress. The findings could provide a theoretical foundation for the healthy cultivation of O. bidens. Three treatment groups were exposed to a mercury concentration of 0.107 mg/L (one-third of the 96h LD₅₀) and fed diets containing different levels of S. platensis: 0% (mercury stress group), 4% (mercury + 4% algae group), or 8% (mercury + 8% algae group). After two weeks of mercury exposure, the weight gain and specific growth rates of O. bidens in the mercury stress group were significantly lower than those in the control group. No significant variations in the weight gain or specific growth rates were observed in the mercury + 4% algae and mercury + 8% algae groups. After two weeks of acclimation, the superoxide dismutase (SOD), catalase (CAT), total antioxidant capacity (T-AOC), acid phosphatase (ACP), alkaline phosphatase (AKP), and lysozyme (LZM) activities in the mercury stress group were significantly lower than those in the control group, while the malondialdehyde (MDA) content showed a significant increase. Except for reduced CAT and T-AOC activities and increased MDA content, no significant differences were observed in SOD, ACP, AKP, and LZM activities between the control and mercury + 4% algae groups. Additionally, no significant variations were observed in any of the antioxidant or immune indicators between the control and the mercury + 8% algae groups. Notably, O. bidens fed 4% or 8% S. platensis exhibited substantial elevations in the expression of HSP70 and HSP90, especially in the 8% S. platensis group. After two weeks of acclimation, HSP70 expression in the mercury + 4% algae and mercury + 8% algae groups was 2.9- and 5.9-fold higher, respectively, while HSP90 expression was 1.5- and 3.5-fold higher, respectively, compared with those in the control group. In summary, exposure to high concentrations of mercury inhibited the growth performance and reduced the antioxidant and immune indicators of O. bidens. However, dietary supplementation with an appropriate amount of S. platensis (especially at 8%) mitigated the growth inhibition, reduced antioxidant capacity, and impaired immune function caused by mercury stress, significantly improving the stress resistance of O. bidens.