Occlusal pits and fissures remain the most vulnerable tooth surfaces for caries initiation, and selecting a sealant that seals these areas reliably is a clinically meaningful question. We investigated the internal adaptation of four pit and fissure sealants—two resin-based and two glass ionomer-based—applied to permanent premolar teeth. Forty extracted premolars were randomly assigned to four groups (n = 10): Group 1, BeautiSealant (BS); Group 2, 3M Clinpro Sealant; Group 3, GC Fuji IX without surface coating; and Group 4, GC Fuji IX with G-Coat Plus. Following 15,000 thermal cycles, internal adaptation was assessed by X-ray microtomography (micro-CT) using silver nitrate as a tracer. No statistically significant difference was found in the overall mean gap volumes between groups (p = 0.774). The 3M™ Clinpro™ Sealant group exhibited the lowest mean gap value (3.45 ± 1.71), followed by BeautiSealant (3.89 ± 1.34) and GC Fuji IX GP^® with G-Coat Plus™ (4.93 ± 4.09), whereas the GC Fuji IX GP^® without coating group showed the highest mean value (5.56 ± 5.35). Resin-based sealants achieved better fissure penetration and marginal adaptation than the glass ionomer formulations tested, though the differences did not reach statistical significance under the conditions of this study.