Background and Aim: One of the weaknesses of Class II composite resin restorations is gingival microleakage which contributes to postoperative sensitivity and secondary caries. The aim was to evaluate the microleakage in Class II composite resin restorations with different thicknesses of resin-modified glass ionomer (RMGI). Materials and Methods: In this in-vitro study, standardized Class II slot cavities were prepared on the proximal surfaces of 90 molars. In group 1, total-etch adhesive and composite resin were applied using the incremental technique. In group 2, total-etch adhesive and composite were applied using the bulk technique. In group 3, 1 mm of RMGI was applied over the gingival floor, which was covered with increments of composite. In group 4, 1 mm of RMGI was placed on the gingival floor and covered with composite using the bulk technique. In group 5, 2 mm of RMGI was applied over the gingival floor, followed by an incremental composite placement. In group 6, 2 mm of RMGI was placed on the gingival floor, and the cavity was filled using the bulk technique. After thermocycling and staining with methylene blue, the samples were sectioned, and the extent of dye penetration was examined under a stereomicroscope. Data were analyzed using Kruskal-Wallis test and logistic regression (α =0. 05). Results: The lowest and highest dye penetrations were observed in the first, second, and fifth groups, respectively. The RMGI thickness did not influence the microleakage scores significantly in either composite placement techniques (P=0. 828). Conclusion: None of the restorative techniques completely eliminated microleakage of Class II composite resin restorations.