Houttuynia cordata is an herbal plant distributed throughout Asia. H. cordata has many bioactive properties, including antibacterial properties. The antibacterial effects of H. cordata on S. mutans remain unknown. Therefore, we treated S. mutans with 1, 3, 5, 10, 20, 30, or 40 mg/mL H. cordata extract at 37°C for 24 h. The antibacterial effect of H. cordata against S. mutans was confirmed using colony forming unit assay and disk diffusion assays. The results of the cell concentration assay demonstrated that H. cordata inhibited the growth of S. mutans in a dose-dependent manner. Prominent growth inhibition was observed after treatment with 10 mg/mL H. cordata extract, and these findings were statistically significant. In addition, no colonies of S. mutans were detected after treatment with 40 mg/mL H. cordata. Disk diffusion assays revealed that 20 mg/mL of H. cordata created a zone of growth inhibition of 11 mm. Therefore, our findings suggest the possibility of using H. cordata in the treatment and prevention of dental caries.