  We report the first observation of the hadronic transition
$\Upsilon(4S)\to\eta'\Upsilon(1S)$, using 496 fb$^{-1}$ data collected at the
$\Upsilon(4S)$ resonance with the Belle detector at the KEKB asymmetric-energy
$e^{+}e^{-}$ collider. We reconstruct the $\eta'$ meson through its decays to
$\rho^0\gamma$ and to $\pi^+\pi^-\eta$, with $\eta\to\gamma\gamma$. We measure:
${\cal B}(\Upsilon(4S)\to\eta'\Upsilon(1S))=(3.43\pm 0.88 {\rm(stat.)} \pm 0.21
{\rm(syst.)})\times10^{-5}$, with a significance of 5.7$\sigma$.
