Chlorhexidine digluconate inhibits oral bacteria and the formation of dental plaque. Protamine sulfate, a polycationic protein, exerts antibacterial activity by altering the cell wall of bacteria. Extracts of Laminaria japonica and Rosmarinus officinalis display antimicrobial effects against oral pathogens. The purpose of this study was to investigate the synergistic effect of chlorhexidine digluconate and protamine sulfate on the inhibitory activity of L. japonica and R. officinalis extracts against Streptococcus mutans, a major etiological agent for dental caries. Minimal inhibitory concentrations (MICs) of chlorhexidine digluconate, protamine sulfate, and L. japonica and R. officinalis extracts were determined by broth dilution method. Synergistic effect of chlorhexidine digluconate or protamine sulfate and extracts of L. japonica or R. officinalis was determined by fractional inhibitory concentration index (FIC). FIC demonstrated the synergistic effects of the different combinations of antibacterial agents. In this study, the use of sub-MIC of chlorhexidine digluconate or protamine sulfate with sub-MIC of L. japonica and R. officinalis extracts resulted in synergistic inhibitory effects of these antibacterial agents except for chlorhexidine digluconate and L. japonica combination.