Streptococcus mutans is the primary etiological agent of dental caries. The antimicrobial peptide $small{D}$-Nal-Pac-525 was designed by replacing the tryptophans of the Trp-rich peptide Pac-525 with $small{D}$-${eta}$-naphthyalanines. To assess the effect of $small{D}$-Nal-Pac-525 on cariogenic bacteria, the activity of $small{D}$-Nal-Pac-525 on the growth of S. mutans and its biofilm formation were examined. $small{D}$-Nal-Pac-525 showed robust antimicrobial activity against S. mutans (minimum inhibitory concentration of 4 ${mu}g/ml$). Using scanning electron microscopy and transmission electron microscopy, it was shown that $small{D}$-Nal-Pac-525 caused morphological changes and damaged the cell membrane of S. mutans. $small{D}$-Nal-Pac-525 inhibited biofilm formation of S. mutans at 2 ${mu}g/ml$. The results of this study suggest that $small{D}$-Nal-Pac-525 has great potential for clinical application as a dental caries-preventing agent.