We present a measurement of the branching fraction B(Ds+→μ+νμ ) using a 548fb-1 data sample collected by the Belle experiment at the KEKB e+e- collider. The Ds momentum is determined by reconstruction of the system recoiling against DKγX in events of the type e+e-→Ds*DKX, Ds*→Dsγ, 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(Ds+→μ+νμ)=[6.44±0.76(stat)±0.57(syst)] ×10-3, implying a Ds decay constant of fDs=[275±16(stat) ±12(syst)]MeV. © 2008 The American Physical Society.