The Beijing Spectrometer (BES) experiment has observed purely leptonic decays of the Ds meson in the reaction e+e-→Ds+Ds- at a c.m. energy of 4.03 GeV. Three events are observed in which one Ds decays hadronically to φπ, K̄*0K, or K̄0K, and the other decays leptonically to μνμ or τντ. With the assumption of μ-τ universality, values of the branching fraction, B(Ds→μνμ)=(1.5-0.6-0.2+1.3+0.3)%, and the Ds pseudoscalar decay constant, fDs=(4.3-1.3-0.4+1.5+0.4)×102 MeV, are obtained. © 1995 The American Physical Society.