Aim <p>The aim of the present in vitro study was to synthesize and characterize green silver nanoparticles using <i>Larrea tridentata</i> leaf extract as a reducing and stabilizing agent and combining the nanoparticles with a commercial fluoride varnish (Fluor Protector^®) to evaluate the antibacterial activity in the production of acid and the adhesion of <i>Streptococcus mutans</i> and <i>Lactobacillus acidophilus.</i></p> Methods <p>Silver nanoparticles were synthesized by green synthesis using <i>Larrea tridentata</i> leaf extract as a reducing and stabilizing agent and characterized by UV–Vis spectroscopy, dynamic light scattering (DLS), and transmission electron microscopy (TEM). Minimum inhibitory and bactericidal concentrations were determined using microdiluted plates. Antibacterial activity compared using disc diffusion. Adhesion and acidogenicity tested on deciduous teeth, divided into experimental (fluoride varnish with silver nanoparticles) and control (fluoride varnish) groups. Microorganism adhesion quantified on enamel, pH readings after 24-h incubation. Analyzed using ANOVA, Tukey’s test, and Mann–Whitney <i>U</i>.</p> Results <p>Fluoride varnish with silver nanoparticles showed better antibacterial activity than nanoparticles and varnish alone. Fluoride varnish with silver nanoparticles was more effective compared to fluoride varnish in inhibiting the adhesion and pH decreases of <i>Streptococcus mutans</i> and <i>Lactobacillus acidophilus</i> on the sound enamel, with statistically significant differences (<i>p</i> &lt; 0.001).</p> Conclusions <p>Fluoride varnish with silver nanoparticles has a good antibacterial effect for inhibiting the adhesion and production of acid being is important in the prevention and the initial stages of dental caries.</p>