  We report a first study of the semileptonic decay $D^0\rightarrow
K^-\pi^0\mu^{+}\nu_{\mu}$ by analyzing an $e^+e^-$ annihilation data sample of
$7.9~\mathrm{fb}^{-1}$ collected at the center-of-mass energy of 3.773 GeV with
the BESIII detector. The absolute branching fraction of $D^0\to
K^-\pi^0\mu^{+}\nu_{\mu}$ is measured for the first time to be $(0.729 \pm
0.014_{\rm stat} \pm 0.011_{\rm syst})\%$. Based on an amplitude analysis, the
$S\text{-}{\rm wave}$ contribution is determined to be $(5.76 \pm 0.35_{\rm
stat} \pm 0.29_{\rm syst})\%$ of the total decay rate in addition to the
dominated $K^{*}(892)^-$ component. The branching fraction of $D^0\to
K^{*}(892)^-\mu^+\nu_\mu$ is given to be $(2.062 \pm 0.039_{\rm stat} \pm
0.032_{\rm syst})\%$, which improves the precision of the world average by a
factor of 5. Combining with the world average of ${\mathcal B}(D^0\to
K^{*}(892)^-e^+\nu_e)$, the ratio of the branching fractions obtained is
$\frac{{\mathcal B}(D^0\to K^{*}(892)^-\mu^+\nu_\mu)}{{\mathcal B}(D^0\to
K^{*}(892)^-e^+\nu_e)} = 0.96\pm0.08$, in agreement with lepton flavor
universality. Furthermore, assuming single-pole dominance parameterization, the
most precise hadronic form factor ratios for $D^0\to K^{*}(892)^{-}
\mu^+\nu_\mu$ are extracted to be $r_{V}=V(0)/A_1(0)=1.37 \pm 0.09_{\rm stat}
\pm 0.03_{\rm syst}$ and $r_{2}=A_2(0)/A_1(0)=0.76 \pm 0.06_{\rm stat} \pm
0.02_{\rm syst}$.
