  This note describes the rediscovery of $B\to\eta' K$ decays in Belle II data,
both in the charged and neutral final state: $B_0\to\eta' K_S$ and
$B^\pm\to\eta' K^\pm$. The $\eta'$ is searched for in two decay modes:
$\eta'\to\eta\pi^+\pi^-$ with $\eta\to\gamma\gamma$, and $\eta'\to\rho\gamma$.
The analysis uses data collected in 2019 and 2020 at the SuperKEKB asymmetric
$e^+e^-$ collider, with an integrated luminosity of $62.8~fb^{-1}$,
corresponding to $68.2$ million of $B\bar{B}$ pairs produced. The signal yield
is obtained via an unbinned maximum likelihood fit to signal sensitive
variables, obtaining branching ratios:
  $$\mathcal{B}\left(B^\pm\to\eta'K^\pm\right) =
\left(63.4~^{+3.4}_{-3.3}\,(stat)\,\pm3.2\,(syst)\,\right) \times10^{-6} $$
  $$\mathcal{B}\left(B_0\to\eta'K_S\right) =
\left(59.9~^{+5.8}_{-5.5}\,(stat)\,\pm2.9\,(syst)\,\right) \times10^{-6} $$
which are consistent with world average.
