<p>Nanoparticles have been applied across various sectors of industry and agriculture. However, concerns regarding environmental contamination and toxicity to target and non-target organisms are increasing. The combination of plant extracts with nanoparticles may enhance their intended effects while reducing overall toxicity. Despite this potential, no studies have evaluated the toxicity of a nanocomposite based on plant extract and silver nanoparticles (Ag Np) on the survival and hematological parameters of tambaqui (<i>Colossoma macropomum</i>). This study assessed the acute toxicity and hematological effects of a nanocomposite based on <i>Terminalia catappa</i> extract and Ag Np in tambaqui. Fish (3–5 g) were exposed to five concentrations of the nanocomposite (0.11, 0.22, 0.45, 0.87, and 1.73 mgL⁻¹) in triplicate for 96 h. Moribund fish during exposure period and all survivors at the end of the experiment were subjected to hematological analysis. Mortality data were used to estimate the lethal concentration 50 (LC₅₀) via the parametric probit method. Hematological data were analyzed using ANOVA and Tukey’s test. The nanocomposite exhibited an LC₅₀ of 0.61 mgL⁻¹, classifying it as a highly toxic for tambaqui. All fish exposed to 1.73 mgL⁻¹ died within 2 h. Increases in glucose, plasma proteins, erythrocytes, hemoglobin, MCHC, and additionally, lymphocytopenia, neutrophilia, and monocytosis were observed at 0.87 mgL⁻¹, reflecting a stress and acute respiratory toxicity. However, at lower concentrations (&lt; 0.45 mgL⁻¹) no alterations were observed in hematological and biochemical parameters. In conclusion, the nanocomposite induces hematological alterations consistent with acute stress in high concentrations, nonetheless 0.45 mgL⁻¹ is a critical threshold for fish survival.</p>