Background: This in vitro study aimed to quantitatively evaluate the erosive effects of simulated gastric acid exposure on the surface microhardness of contemporary fissure sealant materials. Methods: A total of 42 cylindrical specimens (4 mm × 2 mm) were fabricated and randomly allocated to six experimental groups (n = 7 per group): Group 1: BeautiSealant (Shofu, Japan), Group 2: Conseal F (SDI, Australia), Group 3: Clinpro (3M ESPE, USA), Group 4: UltraSeal XT Hydro (Ultradent, USA), Group 5: Fissured Nova (Imicryl, Turkey), and Group 6: Riva Protect (SDI, Australia). Specimens were conditioned in distilled water for 24 hours prior to exposure to a standardized 10-day gastric acid cycling regimen, consisting of six daily cycles of 60-second immersion in simulated gastric acid (pH ≈ 1.2) followed by 30-minute remineralization in artificial saliva. Surface microhardness measurements were obtained at baseline and after acid exposure using a Vickers microhardness tester (Shimadzu, Japan). Results: Significant intergroup differences were observed in baseline microhardness values (p < 0.05). No statistically significant difference was found between the groups in their post-exposure microhardness values (p > 0.05). These findings demonstrate a convergence in material behavior following simulated gastric acid exposure. All materials demonstrated a significant decline relative to baseline (p < 0.05), indicating susceptibility to acid-induced degradation. Notably, despite significant differences at baseline, post-exposure microhardness values converged across all material groups, indicating a uniform softening effect following repeated acid challenge. Conclusions: All tested fissure sealants showed reduced microhardness after repeated gastric acid exposure, indicating limited resistance to highly acidic conditions and potential vulnerability in patients with erosion-related conditions.