Streptococcus mutans, a typical dental caries-causing oral pathogen, forms dental biofilm by attaching to the surface of teeth. Its glucosyltransferase (GTFase) is responsible for synthesizing water-insoluble glucan from sucrose, which is an important factor for biofilm formation and for providing microbial resistance against stresses. Therefore, inhibiting the activity of GTFase is an effective approach to prevent the formation of dental biofilm even without killing S. mutans. In this study, we found that 2.5 mg/mL of methanol extracts of Camellia sinensis leaf, Diospyros kaki leaf, Nelumbo nucifera seed, Rubus coreanus fruit, and Ulmus davidiana rhizodermis inhibits both GTFase activity and biofilm formation of S. mutans without affecting cell growth. Phenolic acids such as ferulic acid, salicylic acid, and vanillic acid in R. coreanus fruit extract inhibited GTFase activity. Enzyme kinetic analysis shows that ferulic acid, salicylic acid, and vanillic acid are competitive, noncompetitive, and uncompetitive inhibitors, respectively. These results suggest that R. coreanus fruit extracts containing phenolic acids may control and prevent dental caries without killing S. mutans by inhibiting GTFase activity.