Lepton flavor universality in rare b→s transitions is tested for the first time using B_{s}^{0} meson decays. The measurements are performed using pp collision data collected by the LHCb experiment between 2011 and 2018, corresponding to a total integrated luminosity of 9 fb^{-1}. Branching fraction ratios between the B_{s}^{0}→ϕe^{+}e^{-} and B_{s}^{0}→ϕμ^{+}μ^{-} decays are measured in three regions of dilepton mass squared, q^{2}, with 0.1<q^{2}<1.1, 1.1<q^{2}<6.0, and 15<q^{2}<19 GeV^{2}/c^{4}. The results agree with the standard model expectation of lepton flavor universality.