Purpose: The aim of this study was to determine the antimicrobial effects of propolis extracted from an endemic species of stingless bee, T. sirindhornae, on the cariogenic bacterium Streptococcus mutans. Materials and Methods: Dichloromethane extracts (DME) of propolis (DMEP) were prepared and analysed by reverse-phase high-performance liquid chromatography. The antibacterial growth and antibiofilm formation effects of DMEP on S. mutans were compared with those of apigenin, a commercial propolis product. The effects of DMEP and apigenin on glucosyltransferase (gtf) B expression in S. mutans were investigated using real-time polymerase chain reaction. Chlorhexidine (CHX) was used as a positive control in the experiments. Results: Apigenin, pinocembrin, p-coumaric acid, and caffeic acid were not detected in the propolis extracts. DMEP and apigenin significantly inhibited S. mutans growth (IC50 = 43.5 and 17.36 mg/ml, respectively). DMEP and apigenin also exhibited antiadherence effects on S. mutans as shown by reduced biofilm formation. Furthermore, a significant inhibition in gtfB expression was observed in DMEP and apigenin treated S. mutans. Conclusion: Propolis produced by T. sirindhornae demonstrated antibacterial and antibiofilm effects, and reduced gtfB expression in S. mutans. The antibacterial activities of propolis observed were not due to apigenin, pinocembrin, p-coumaric acid, or caffeic acid. Keywords: biofilm, glucosyltransferases, propolis, Streptococcus mutans, Trigona sirindhornae