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In this study, the biosynthesized silver nanoparticle composites (AgNPCs) obtained from the polysaccharide extract of the red alga Calliblepharis fimbriatawere used to control the infection of Xanthomonas oryzaepv. oryzaebacteria causing blight disease in rice. The effect of AgNPCs was evaluated based on seed germination, growth parameters, and photosynthetic pigments of the Oryzae sativa. The formation of AgNPCs was monitored by UV–visible spectrophotometer and characterized by using XRD, SEM, EDAX, and TEM analysis. The synthesized AgNPCs appear spherical in shape, and the size was measured in the range of 25–30 nm. Further, the aqueous extract of seaweed derived AgNPCs was characterized using FTIR, 1D and 2D 1H NMR, and GC-MS analysis. Among the different treatments, a AgNPCs concentration of 50 μg/mL was found to show a stimulatory effect in seed germination and its related growth parameters over control and untreated seeds. In this study, AgNPCs controlled the bacterial colonization and also enhanced the biochemical characteristics of infected rice plants. Later, EDAX analysis showed that infected and uninfected rice seedlings treated with 50 μg/mL of AgNPCs were free of Ag particles but increased amount of nutrients like P, K, C, S, Mg, and Cl in plants. However, reduction in the Ca and Fe ion concentrations was not below the level of untreated plants. This study concluded that AgNPCs of red alga C. fimbriatahave growth stimulation as well as pesticidal properties to control bacterial blight in rice without compromising host growth, a potential substrate for nanopesticide production.