interface*

Single-layer FeSe on a SrTiO₃ substrate, with its extraordinarily large T_c amongst all interfacial superconductors and iron based superconductors, is particularly interesting, but the mechanism underlying its high T_c has remained mysterious. Here we show through isotope effects that electrons in FeSe couple with the oxygen phonons in the substrate, and the superconductivity is enhanced linearly with the coupling strength atop the intrinsic superconductivity of optimally-electron-doped FeSe. Our observations solve the enigma of FeSe/SrTiO₃, and experimentally establish the critical role and unique behavior of electron-phonon forward scattering in a correlated high-T_c superconductor. The effective cooperation between interlayer electron-phonon interactions and correlations suggests a path forward in developing more high-T_c interfacial superconductors, and may shed light on understanding the high T_c of bulk high temperature superconductors with layered structures.