  We present an analysis of the charmless semileptonic decay $B^0\to\pi^-
\ell^+ \nu_{\ell}$, where $\ell = e, \mu$, from 198.0 million pairs of
$B\bar{B}$ mesons recorded by the Belle II detector at the SuperKEKB
electron-positron collider. The decay is reconstructed without identifying the
partner $B$ meson. The partial branching fractions are measured independently
for $B^0\to\pi^- e^+ \nu_{e}$ and $B^0\to\pi^- \mu^+ \nu_{\mu}$ as functions of
$q^{2}$ (momentum transfer squared), using 3896 $B^0\to\pi^- e^+ \nu_{e}$ and
5466 $B^0\to\pi^- \mu^+ \nu_{\mu}$ decays. The total branching fraction is
found to be $(1.426 \pm 0.056 \pm 0.125) \times 10^{-4}$ for $B^0\to\pi^-
\ell^+ \nu_{\ell}$ decays, where the uncertainties are statistical and
systematic, respectively. By fitting the measured partial branching fractions
as functions of $q^{2}$, together with constraints on the nonperturbative
hadronic contribution from lattice QCD calculations, the magnitude of the
Cabibbo-Kobayashi-Maskawa matrix element $V_{ub}$, $(3.55 \pm 0.12 \pm 0.13 \pm
0.17) \times 10^{-3}$, is extracted. Here, the first uncertainty is
statistical, the second is systematic and the third is theoretical.
