We present the latest Belle II and Belle results on rare $B$ decays, based on data samples of $365$ and $711\,\mathrm{fb}^{-1}$, respectively, collected at the $\Upsilon(4S)$ resonance. Rare $B$ decays, in particular flavor-changing neutral currents (FCNC) of the type $b \to s$, provide unique sensitivity to a wide range of new-physics scenarios. Belle II and Belle are achieving world-leading precision and are probing intriguing anomalies in $B^+ \to K^+ \nu\bar{\nu}$ that have generated broad interest within the particle-physics community. We discuss several new FCNC measurements, as well as results on $B^+ \to \mu^+ \nu_\mu$, which set the current most stringent limits on branching fractions and impose new constraints on theoretical models.