  We report the first observation of the radiative decay of the $\Upsilon(1S)$
into a charmonium state. The statistical significance of the observed signal of
$\Upsilon(1S) \to \gamma \chi_{c1}$ is 6.3 standard deviations including
systematics. The branching fraction is calculated to be Br($\Upsilon(1S) \to
\gamma \chi_{c1}$) = (4.7^{+2.4}_{-1.8} (stat) ^{+0.4}_{-0.5} (sys)) * 10^{-5}.
We also searched for $\Upsilon(1S)$ radiative decays into $\chi_{c0,2}$ and
$\eta_c(1S,2S)$ and set upper limits on their branching fractions. These
results are obtained from a 24.9 fb^{-1} data sample collected with the Belle
detector at the KEKB asymmetric-energy $e^+e^-$ collider at a center-of-mass
energy equal to the $\Upsilon(2S)$ mass using $\Upsilon(1S)$ tagging by the
$\Upsilon(2S) \to \Upsilon(1S) \pi^+\pi^-$ transitions.
