Dental caries is an important global health concern and Streptococcus mutans has been established as a major cariogenic bacterial species. Reports indicate that a rare sugar, D‑tagatose, is not easily catabolized by pathogenic bacteria. In the present study, the inhibitory effects of D‑tagatose on the growth and biofilm formation of S. mutans GS‑5 were examined. Monitoring S. mutans growth over a 24 h period revealed that D‑tagatose prolonged the lag phase without interfering with the final cell yield. This growth retardation was also observed in the presence of 1% sucrose, although it was abolished by the addition of D‑fructose. S. mutans biofilm formation was significantly inhibited by growth in sucrose media supplemented with 1 and 4% D‑tagatose compared with that in a culture containing sucrose alone, while S. mutans formed granular biofilms in the presence of this rare sugar. The inhibitory effect of D‑tagatose on S. mutans biofilm formation was significantly more evident than that of xylitol. Growth in sucrose media supplemented with D‑tagatose significantly decreased the expression of glucosyltransferase, exo‑β‑fructosidase and D‑fructose‑specific phosphotransferase genes but not the expression of fructosyltransferase compared with the culture containing sucrose only. The activity of cell‑associated glucosyltransferase in S. mutans was inhibited by 4% D‑tagatose. These results indicate that D‑tagatose reduces water‑insoluble glucan production from sucrose by inhibiting glucosyltransferase activities, which limits access to the free D‑fructose released during this process and retards the growth of S. mutans. Therefore, foods and oral care products containing D‑tagatose are anticipated to reduce the risk of caries by inhibiting S. mutans biofilm formation.