The polarization of photons produced in radiative Bs0 decays is studied for the first time. The data are recorded by the LHCb experiment in pp collisions corresponding to an integrated luminosity of 3 fb-1 at center-of-mass energies of 7 and 8 TeV. A time-dependent analysis of the Bs0→φγ decay rate is conducted to determine the parameter AΔ, which is related to the ratio of right- over left-handed photon polarization amplitudes in b→sγ transitions. A value of AΔ=-0.98-0.52+0.46-0.20+0.23 is measured. This result is consistent with the standard model prediction within 2 standard deviations.