We report the first (to our knowledge) experimental observation of resonant cavity-enhanced photosensitivity in ${\mathrm{As}}_2{\mathrm{S}}_3$ chalcogenide glass film at $1550\mathrm{nm}$ telecommunication wavelength. The measured photosensitivity threshold is $<0.1\mathrm{GW}∕{\mathrm{cm}}^2$ , and a photoinduced refractive index increase as large as 0.016 is observed. The photosensitive process is athermal; further, we confirm the absence of two-photon absorption in ${\mathrm{As}}_2{\mathrm{S}}_3$ , suggesting that defect absorption accounts for the energy transfer from photons to glass network. Besides its potential application for reconfigurable photonics circuit, such photosensitivity is also an important design consideration for nonlinear optical devices using chalcogenide glasses.