Objective: This in vitro study compared the microleakage of resin composite restorations placed in Class II cavity preparations using an incremental placement technique with and without the addition of precured resin composite inserts. Method and materials: Sixty standardized mesio-occlusal and disto-occlusal cavities were prepared in 30 recently extracted molar teeth. Twenty cavities were restored by employing the incremental placement technique (group I), and 40 cavities were restored using the incremental placement in the presence of precured composite inserts of two sizes: 1X 3 mm (group II) and 1.25 X 3 mm (group III). The restored teeth were thermocycled for 250 cycles in water baths set at temperatures of 5°C and 50°C using a 30-second dwell time, stained, and sectioned horizontally at the middle of the restoration. Following sectioning, each specimen was microscopically examined at X4 magnification. The extent of dye penetration at the tooth/restoration interface was scored for each test group, and the scores obtained were subjected to one-way analysis of variance (ANOVA) and Tukey's pairwise comparison at a 95% significance level. Results: The current investigation showed statistically significant differences in dye penetration between the three test groups. Conclusion: The use of precured resin composite inserts with the incremental placement technique in the restoration of Class II cavity preparations seems to improve the marginal seal of the proximal walls of finished restorations.