Interface-enhanced superconductivity in single-unit-cell FeSe on ${\mathrm{SrTiO}}_3$ has been extensively pursued recently. The interfacial electron-phonon coupling (EPC) is widely proposed to enhance pairing, yet to be directly verified. Herein, using ultrafast pump-probe spectroscopy, we discover a coherent 4.2 THz optical phonon mode that emerges only upon photoexcitation in FeTe/FeSe/${\mathrm{SrTiO}}_3$ or FeTe/${\mathrm{SrTiO}}_3$ heterostructures, but is absent in bare Nb-doped ${\mathrm{SrTiO}}_3$ under identical subgap pumping. Atomic-scale electron energy loss spectroscopy in scanning transmission electron microscopy identifies this mode as out-of-plane oxygen vibrations localized at double ${\mathrm{TiO}}_x$ terminated interface. Crucially, the phonon amplitude in FeTe/FeSe/${\mathrm{SrTiO}}_3$ is more than twice as large as that in FeTe/${\mathrm{SrTiO}}_3$, which can be attributed to higher electron doping that strengthens dipole moments penetrating the FeSe layer, thereby resulting in an enhanced interfacial EPC. Temperature-dependent dynamics further highlight the unique electronic nature of monolayer FeSe in sustaining this robust coupling. These findings provide direct evidence for dipole-mediated interfacial EPC as a critical mechanism of interfacial superconductivity in $\mathrm{FeSe}/{\text{SrTiO}}_3$.

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• Superconductors
• Iron-based superconductors
• Laser techniques