Abstract: Improving the wet interfacial adhesion between epoxy resins and cementitious materials is critical for enhancing the environmental adaptability of repair technologies. Inspired by the strong adhesion of barnacles, this study proposes a wet-interface adhesion enhancement strategy based on hydrophobic interactions. BSEM, XPS, and ITC characterizations, together with dry/wet interfacial bond strength tests, were employed to systematically investigate the effects of silane-modified epoxy prepolymer (Si-PPM) on the micro-interfacial thickness, intermolecular interactions, and thermodynamic driving forces between epoxy resin and cementitious materials. The results show that the incorporation of Si-PPM reduced the polar component of the surface free energy of the cured epoxy from 10.9 mN·m⁻¹ to 5.4 mN·m⁻¹. Owing to the decrease in the intrinsic mechanical properties of the cured epoxy, the bond strength of the Si-PPM modified system at the dry interface decreased from 7.2 MPa to 5.9 MPa. In contrast, the strength at the wet interface increased from 4.0 MPa to 5.5 MPa, showing a 37.5% improvement and approaching the strength level under dry conditions. This improvement is mainly attributed to the exclusion of interfacial water molecules induced by hydrophobic silane segments, which increases solvation entropy and thereby promotes effective wetting and intimate contact at the wet interface. Meanwhile, the introduction of Si-PPM facilitates the local enrichment of aromatic structures in the interfacial region, thereby enhancing π-π interactions. In addition, retained polar groups such as ether bonds form hydrogen bonds with silanol and calcium hydroxyl sites on the CSH surface, further improving the stability of wet interfacial adhesion.