The green synthesis of silver nanoparticles (AgNPs) using marine macroalgae has attracted increasing attention as an environmentally friendly alternative to conventional chemical and physical synthesis methods because it reduces the use of hazardous chemicals and minimizes environmental impacts. However, information regarding the acute toxicity and immunomodulatory effects of biosynthesized AgNPs in fish remains limited. Therefore, this study aimed to biosynthesize AgNPs using Sargassum angustifolium extract and evaluate their acute toxicity and effects on selected innate immune responses in Barbus xanthopterus. Silver nanoparticles were biosynthesized by reducing a 1 mM silver nitrate (AgNO₃) solution with S. angustifolium extract. The 96-h median lethal concentration (LC₅₀) was determined according to the OECD (1998) guideline, after which fish were exposed for 21 days to sublethal concentrations corresponding to 10, 25, 50, and 75% of the LC₅₀. The 96-h LC₅₀ value of the biosynthesized AgNPs for B. xanthopterus was 4.98 mg/L. Serum lysozyme activity and respiratory burst activity were not significantly affected by AgNP exposure compared with the control group (p>0.05). In contrast, serum complement activity was significantly lower in fish exposed to 75% of the 96-h LC₅₀ than in the other treatment groups (p<0.05), with the lowest value observed at the final sampling time. These findings suggest that although biosynthesized AgNPs had no significant effects on serum lysozyme activity or respiratory burst activity, exposure to higher sublethal concentrations may impair complement-mediated innate immunity in B. xanthopterus, highlighting the importance of evaluating the ecological safety of green-synthesized AgNPs before their widespread application in aquatic environments.