Background <p>Restoring extensively damaged posterior teeth remains challenging due to reduced fracture resistance and potential marginal deterioration over time. Contemporary composite resins, including fiber-reinforced, injectable, and conventional materials, have been developed to improve mechanical performance and marginal integrity. This in vitro study evaluated the effect of thermo-mechanical aging on marginal adaptation and fracture resistance of these composite resins materials used in extensive Class II MOD cavities.</p> Methods <p>Sixty-five extracted mandibular molars were allocated to five groups (<i>n</i> = 13): intact control, G-ænial Posterior, GAP: G-ænial Universal Injectable, EXF: EverX Flow, or EXP: EverX Posterior (GC, Japan). Standardized MOD cavities were prepared, selectively etched, and restored using G-Premio Bond (GC, Japan). Aging was simulated using thermocycling and chewing simulation. Marginal adaptation was assessed by scanning electron microscopy before and after aging, and fracture resistance was tested after aging. Data were analyzed using two-way ANOVA, one-way ANOVA, Tukey’s post-hoc, and Chi-square tests (<i>p</i> &lt; 0.05).</p> Results <p>Marginal adaptation differed significantly among materials both before and after aging (<i>p</i> &lt; 0.05). Aging increased marginal gaps in all groups, with the highest values in GUI and the lowest in EXP. EXF showed higher fracture resistance than GUI (<i>p</i> &lt; 0.05). Restorability was highest for EXP and lowest for GAP, although the difference was not significant.</p> Conclusion <p>Thermomechanical aging increased marginal gaps in all materials. EXF and EXP, showed more favorable marginal adaptation and higher fracture resistance than GUI and GAP. These materials may be considered a suitable option for restoring extensively damaged posterior teeth.</p>