  We report a measurement of the branching fraction $B(D^+ \rightarrow \mu^+
\nu_{\mu}) = [3.71 \pm 0.19 (\rm stat) \pm 0.06 (\rm sys)]\times 10^{-4}$ based
on 2.92 ${\rm fb^{-1}}$ of data accumulated at $\sqrt{s}=3.773$ GeV with the
BESIII detector at the BEPCII collider. This measurement, in conjunction with
the Cabibbo-Kobayashi-Maskawa matrix element $|V_{\rm cd}|$ determined from a
global Standard Model fit, implies a value for the weak decay constant
$f_{D^+}=(203.2 \pm 5.3 \pm 1.8)$ MeV. Additionally, using this branching
fraction measurement together with a Lattice QCD prediction for $f_{D^+}$, we
find $|V_{\rm cd}|=0.2210\pm 0.0058 \pm 0.0047$. In either case, these are the
most precise results for these quantities to date.
