This study aimed to evaluate the polymerization depths of two different bulk-fill posterior composite resins and a conventional posterior composite resin with the microhardness test and there with the in vitro cytotoxicity test. For the microhardness test, 10 samples were obtained for each group using Teflon molds. Microhardness values were measured from the top and bottom surfaces of each sample using the Vicker's microhardness test. For the cytotoxicity test, 12 samples were obtained for each group using Teflon molds. The extraction fluids of samples were obtained by incubating them at 37°C in RPMI 1640 medium for 24, 48, and 72 h. Cell viability was assessed using the MTT assay following incubation. The microhardness values of the top and bottom surfaces of composite resins were aligned as follows: Filtek Z550 Composite Resin > Filtek Bulk-Fill Posterior Composite Resin > Tetric N-Ceram Bulk-Fill Composite Resin. The microhardness ratio of Filtek Z550 Composite Resin was found to be significantly higher than those of other composite resins when the bottom surface–top surface microhardness ratios were compared. The difference between the bottom surface–top surface microhardness ratios of the Filtek Bulk-Fill Posterior Composite Resin and the Tetric N-Ceram Bulk-Fill Composite Resin was not statistically significant. However, all composite resins provided the recommended polymerization depth (≥80%). No statistically significant difference was observed between the cytotoxicity values of the composite resins after 24- and 48-h incubation. However, the Filtek Bulk-Fill Posterior Composite Resin showed a significantly higher cytotoxicity compared with the other groups after 72-h incubation. In addition, the cytotoxicity value obtained after 72-h incubation of the Filtek Bulk-Fill Posterior Composite Resin was significantly higher than the cytotoxicity values obtained after 24- and 48-h incubation. Although adequate polymerization depth was obtained in all study groups, the Filtek Bulk-Fill Posterior Composite Resin had significantly reduced cell viability after 72-h incubation.