Objectives: This study evaluated the clinical performance of bulk-fill resin composite systems with different viscosities in class II cavities. Materials and Methods: A total of 80 class II restorations were performed with a sin- gle operator in 50 patients using four different bulk-fill resin composite materials: Fil- tekTM Bulk Fill, Heated FiltekTM Bulk Fill, G-ænialTM BULK Injectable, and SonicFill3. A double-blinded randomized clinical trial was designed to evaluate the two-year clini- cal performance of the four bulk-fill composite resins using modified FDI criteria. Data were analyzed descriptively. Level of significance was set at 0.05. Differences between groups were tested using Wilcoxon-signed-rank and Mann–Whitney-U test as nonparametric tests. Results: Data were analyzed using Kruskal-Wallis, Mann–Whitney U, and Wilcoxon signed rank tests (p < 0.05). Eighty restorations were available for evaluation, with a survival function of 100%. No statistically significant differences were observed between the four groups regarding assessing esthetic, functional, and biological prop- erties during different follow-up periods. Conclusions: After 2 years of clinical follow-up, the bulk-fill composite systems with different viscosities seem to be esthetically, functionally, and biologically acceptable, with a promising clinical performance in class II cavities.