This study evaluated the antimicrobial capacity of BlueM® mouthwash against the bacterium Streptococcus mutans and its influence on gbpA gene expression as well as its cytotoxic effect on fibroblast cells. BlueM® showed antimicrobial activity, with MIC and MBC values of 0.005% and 0.01%, respectively. The MBIC was 6.25% for S. mutans. CFU count and confocal microscopy revealed significant effect of BlueM® on S. mutans biofilm pre-formed on dentin surfaces. Interestingly, the analysis of gbpA gene expression indicated a decrease in gene expression after 15 min of treatment with BlueM® at a concentration of 25%. Moreover, BlueM® exhibited low levels of cytotoxicity. In conclusion, our results showed the antimicrobial effectiveness of BlueM® against S. mutans, its ability to modulate the expression of the gbpA gene and its low cytotoxicity. This study supports the therapeutic potential of BlueM® as an alternative agent for the control of oral biofilm.