Ag/SiO₂ colloidal nanocomposites (NCs) were prepared through the semi-continuous chemical reduction of silver ions on a silica surface; NaBH4 was used as a primary reducing agent, while carboxymethyl cellulose (CMC) served as a secondary reductant and a stabilizer at low temperature. Silver nanoparticles (AgNPs) of an average diameter of 3.89±0.18 nm were uniformly and densely dispersed on the SiO₂ surface, forming 218.6-nm-sized Ag/SiO₂ NCs. The zeta potential of the Ag/SiO₂ NCs (-92.6 mV) was more negative than that of silica (-24 mV), indicating their high long-term stability. Furthermore, their proposed formation mechanism was confirmed via Fourier transform infrared spectroscopy. Then, the bactericidal effect of the Ag/SiO₂ was evaluated based on their minimal inhibitory concentration (MIC) against Ralstonia solanacearum 15 (R. solanacearum 15); it was 62.5 ppm, much lower than that of conventional AgNPs (500 ppm). Therefore, these highly stable Ag/SiO₂ colloidal NCs with more effective antibacterial activity than conventional AgNPs are a promising nanopesticide in agriculture.