© 2019 CERN, for the LHCb Collaboration. Published by the American Physical Society under the terms of the "https://creativecommons.org/licenses/by/4.0/"Creative Commons Attribution 4.0 International license. Further distribution of this work must maintain attribution to the author(s) and the published article's title, journal citation, and DOI. Funded by SCOAP 3 . A time-dependent analysis of the Bs0→φγ decay rate is performed to determine the CP -violating observables Sφγ and Cφγ and the mixing-induced observable AφγΔ. The measurement is based on a sample of pp collision data recorded with the LHCb detector, corresponding to an integrated luminosity of 3 fb-1 at center-of-mass energies of 7 and 8 TeV. The measured values are Sφγ=0.43±0.30±0.11, Cφγ=0.11±0.29±0.11, and AφγΔ=-0.67-0.41+0.37±0.17, where the first uncertainty is statistical and the second systematic. This is the first measurement of the observables S and C in radiative Bs0 decays. The results are consistent with the standard model predictions.