  We report the observation of $\Upsilon(2S)\to\gamma\eta_{b}(1S)$ decay based
on analysis of the inclusive photon spectrum of $24.7$ fb$^{-1}$ of $e^+ e^-$
collisions at the $\Upsilon(2S)$ center-of-mass energy collected with the Belle
detector at the KEKB asymmetric-energy $e^+ e^-$ collider. We measure a
branching fraction of
$\mathcal{B}(\Upsilon(2S)\to\gamma\eta_{b}(1S))=(6.1^{+0.6+0.9}_{-0.7-0.6})\times
10^{-4}$, and derive an $\eta_{b}(1S)$ mass of $9394.8^{+2.7+4.5}_{-3.1-2.7}$
MeV/$c^{2}$, where the uncertainties are statistical and systematic,
respectively. The significance of our measurement is greater than 7 standard
deviations, constituting the first observation of this decay mode.
