[en] We present a measurement of the branching fraction B(D_s⁺→μ⁺ν_μ) using a 548 fb^-1 data sample collected by the Belle experiment at the KEKB e⁺e^- collider. The D_s momentum is determined by reconstruction of the system recoiling against DKγX in events of the type e⁺e^-→D_s*DKX, D_s*→D_sγ, where X represents additional pions or photons from fragmentation. This full-reconstruction method provides high resolution in the neutrino momentum and thus good background separation, equivalent to that achieved by experiments at the tau-charm factories. We obtain the branching fraction B(D_s⁺→μ⁺ν_μ)=[6.44±0.76(stat)±0.57(syst)]x10^-3, implying a D_s decay constant of f_Ds=[275±16(stat)±12(syst)] MeV