Aim: To assess the anticariogenicity of microwave-assisted 0.5% extract of Stevia rebaudiana leaves in high caries risk patients. Materials and methods: Forty-six patients with high risk for caries were selected. They were randomly assigned to two groups; group I was allocated for chlorhexidine (CHX) mouthwash (0.12% Perio-Aid) and group II for S. rebaudiana (0.5% extract of Stevia bio) mouthwash. Salivary pH, buffer, and microbial count were assessed before the patients were asked to use the mouthwashes. Patients were prescribed the mouthwash/ extract twice a day for 7 days. On the 8th day, postrinse salivary pH, buffer analysis and Streptococcus mutans and Lactobacilli count were done. Results: Significant difference in pre- and postrinse values of pH and buffer was found in both groups. However, no difference was noted between groups. Both groups decreased the bacterial count to <105 colony-forming units (CFUs) in all the patients. Conclusion: Stevia rebaudiana extract in 0.5% concentration improved the pH and buffering capacity of the saliva in a high caries risk patient. It also reduced cariogenic organisms in saliva. Clinical significance: Stevia rebaudiana extract in 0.5% concentration can be used as a mouthwash for moderate-to-high caries risk patients. However, long-term clinical studies are required to prove its substantivity like that of CHX.