Using (2712.4±14.3)×10^{6} ψ(3686) events collected with the BESIII detector at the BEPCII collider, the decay η_{c}→γγ in J/ψ→γη_{c} is observed. We determine the product branching fraction B(J/ψ→γη_{c})×B(η_{c}→γγ)=(5.23±0.26_{stat}±0.30_{syst})×10^{-6}. This result is consistent with the lattice QCD calculation (5.34±0.16)×10^{-6} from HPQCD in 2023. By using the world-average values of B(J/ψ→γη_{c}) and the total decay width of η_{c}, the partial decay width Γ(η_{c}→γγ) is determined to be (11.30±0.56_{stat}±0.66_{syst}±1.14_{ref}) keV, which deviates from the corresponding world-average value by 3.4σ.