  We report on the Imaging X-ray Polarimetry Explorer (IXPE) observation of the
closest and X-ray brightest Compton-thick active galactic nucleus (AGN), the
Circinus galaxy. We find the source to be significantly polarized in the 2--6
keV band. From previous studies, the X-ray spectrum is known to be dominated by
reflection components, both neutral (torus) and ionized (ionization cones). Our
analysis indicates that the polarization degree is $28 \pm 7$ per cent (at 68
per cent confidence level) for the neutral reflector, with a polarization angle
of $18{\deg} \pm 5{\deg}$, roughly perpendicular to the radio jet. The
polarization of the ionized reflection is unconstrained. A comparison with
Monte Carlo simulations of the polarization expected from the torus shows that
the neutral reflector is consistent with being an equatorial torus with a
half-opening angle of 45{\deg}-55{\deg}. This is the first X-ray polarization
detection in a Seyfert galaxy, demonstrating the power of X-ray polarimetry in
probing the geometry of the circumnuclear regions of AGNs, and confirming the
basic predictions of standard Unification Models.
