Background: Streptococcus mutans is the most cariogenic microorganism with high adherence ability to the tooth surfaces. The probiotics have attracted attention as a new and bioecological technique to inhibit oral bacterial colonization. Enterococcal strains, such as Enterococcus durans, are abundant in nature and can be identified as a probiotic. Objectives: Since the reduction of adhesion can be an effective way to decrease the cariogenic potential of S. mutans, the present study aimed to evaluate the inhibitory effect of E. durans on the adhesion of S. mutans. Methods: In this in vitro study, the standard strain bacteria of probiotic E. durans and Streptococcus mutans and 12 clinical samples of S. mutans were used. The ability of S. mutans biofilm formation was assessed. Then, the effect of E. durans on S. mutans adhesion was determined via microtiter plate technique by two methods: (1) Adding a mixed suspension of E. durans and S. mutans simultaneously; (2) adding E. durans 30 minutes before the inoculation of S. mutans to the system. The data were analyzed with SPSS 20 by using paired t-test, and statistical significance was set at P < 0.05. Results: The results showed a decrease in adhesion in the presence of E. durans, with the use of both methods (P < 0.05). Conclusions: Based on the results of this in vitro study, the use of probiotic E. durans decreased the adhesion of S. mutans.