Introduction: The G-aenial Universal Injectable, a new resin composite for tooth and wear restoration, is under investigation for its potential impact on microhardness due to stomach acid.Objective: This in vitro study aimed to evaluate the surface microhardness of universal injectables and investigate the impact of hydrochloric acid (HCL) and thermal cycling on microhardness. Materials and methods: Eight (G-aenial Universal—G-aenial Universal Injectable, GC)—and eight (3M Filtek Universal—3M Filtek Z350 XT Universal Restorative, 3M) specimens with a total number of 32 specimens (2 × 10 mm) were used. Based on the aging condition, the tested material was divided into two subgroups (n = 8): one was kept in deionized water (Water) (pH = 7) at 37°C for 24 hours, and the other in HCL solution (pH = 1.2) for 3 hours. Following light curing, finishing, polishing, immersion in HCL, and thermal cycling, a digital display Vickers microhardness tester is used to evaluate the surface microhardness. Results: The study found a significant difference in Vickers microhardness between G-aenial Universal and 3M Filtek Universal for water (p < 0.001), with G-aenial Universal having the highest microhardness and 3M Filtek Universal having the lowest. No significant difference was found for HCL+TC composites. However, there was a significant difference in Vickers microhardness among aging conditions, with Water having the highest microhardness. Conclusions: The G-aenial Universal can replace 3M Feltik Universal for healthy patients with near-neutral oral conditions, but not for recurrent gastric reflux patients due to their preferred microhardness.