The aim of this in-vitro study is to evaluate the effect of potassium iodide (KI) on erythrosine-mediated photodynamic therapy (PDT) against Streptococcus mutans biofilms. S. mutans ATCC 25175 was cultured to form a biofilm on a hydroxyapatite disk. After diluting erythrosine to 20 μM and KI to 10, 50, and 100 mM, respectively, PDT was performed. The number of surviving bacteria was calculated as colony forming units (CFU)/mL and the statistical significance of the difference between groups was confirmed by Bonferroni post-hoc analysis. Cell viability was visually evaluated using confocal laser scanning microscopy (CLSM). As a result of the experiment, a significant decrease (p < 0.05) in CFU was observed in the experimental groups in which PDT was performed after applying KI regardless of the concentration of KI. In addition, a significant reduction (p < 0.05) in CFU was observed in the experimental group to which 100 mM KI was applied compared to 10 mM KI. The same results were confirmed when observing CLSM. KI significantly improved the efficacy of erythrosine-mediated PDT on S. mutans biofilms at all concentrations. This may compensate for the low sensitivity of PDT to biofilm-state bacteria strains, but it is necessary to establish an optimal clinical protocol through further research.