With short-fiber reinforced resin composite (SFRC), improved mechanical strength and good workability could be achieved within a single material. However, there remains concerns related to microleakage, which could restrict the application of this material in certain clinical situations. Thus, the aim of the study is to investigate microleakage of class II cavities restored with SFRC compared to other resin composites, in a simulated aging environment using thermocycling. Class II cavities were prepared in 80 premolars at the cementoenamel junction. Each group, consisting of 10 specimens, was restored with different materials: bulk-fill flowable SFRC (EverX Flow), bulk-fill SFRC (EverX Posterior), flowable bulk-fill resin composite (Tetric N-flow), and conventional resin composite (Filtek Z350XT). The specimens were divided into two subgroups: one underwent thermocycling of 20,000 cycles, while the other did not. All specimens were subjected to the dye penetration test and then assessed for microleakage scores. In the non-thermocycling group, no significant differences in microleakage scores were observed. In the thermocycling group, EverX Posterior showed a significant difference in microleakage scores compared to Tetric N-flow (p = 0.018) and all other tested materials (all p < 0.001). However, there was no significant difference in microleakage scores between EverX Posterior and Filtek Z350XT (p = 0.714), or between Filtek Z350XT with Tetric N-flow (p = 0.951). In conclusion, when restored with a universal adhesive, bulk-fill flowable SFRC achieved the highest microleakage score compared to other tested resin composite after thermocycling. In addition, all materials showed a significantly higher microleakage score after thermocycling. The materials could be ranked in ascending order of susceptibility to microleakage after aging by bulk-fill SFRC, conventional resin composite, flowable bulk-fill resin composite, and bulk-fill flowable SFRC, respectively.