  The center of the Milky Way Galaxy hosts a $\sim$4 million solar mass black
hole (Sgr A$^*$) that is currently very quiescent with a luminosity many orders
of magnitude below those of active galactic nuclei. Reflection of X-rays from
Sgr A$^*$ by dense gas in the Galactic Center region offers a means to study
its past flaring activity on times scales of hundreds and thousands of years.
The shape of the X-ray continuum and the strong fluorescent iron line observed
from giant molecular clouds in the vicinity of Sgr A$^*$ are consistent with
the reflection scenario. If this interpretation is correct, the reflected
continuum emission should be polarized. Here we report observations of
polarized X-ray emission in the direction of the Galactic center molecular
clouds using the Imaging X-ray Polarimetry Explorer (IXPE). We measure a
polarization degree of 31\% $\pm$ 11\%, and a polarization angle of
$-$48$^\circ$ $\pm$ 11$^\circ$. The polarization angle is consistent with Sgr
A$^*$ being the primary source of the emission, while the polarization degree
implies that some 200 years ago the X-ray luminosity of Sgr A$^*$ was briefly
comparable to a Seyfert galaxy.
