  We present a measurement of the branching fraction and longitudinal
polarization of $B^0 \to \rho^+ \rho^-$ decays. SuperKEKB electron-positron
collision data corresponding to 189~fb$^{-1}$ of integrated luminosity and
containing $198 \times 10^6 B\bar{B}$ pairs collected with the Belle II
detector are used. We obtain \begin{eqnarray*}
  \mathcal{B}(B^0\to\rho^+\rho^-) &=&
[2.67\pm0.28\,(\mathrm{stat})\,\pm0.28\,(\mathrm{syst})] \times 10^{-5},
\end{eqnarray*} \begin{eqnarray*}
  f_L &=& 0.956\pm0.035\,(\mathrm{stat})\,\pm 0.033\,(\mathrm{syst}),
\end{eqnarray*} These results are consistent with previous measurements and can
be used to constrain penguin pollution and to extract the quark-mixing angle
$\phi_2$.
