  We present an X-ray spectro-polarimetric analysis of the bright Seyfert
galaxy NGC4151. The source has been observed with the Imaging X-ray Polarimetry
Explorer (IXPE) for 700 ks, complemented with simultaneous XMM-Newton (50 ks)
and NuSTAR (100 ks) pointings. A polarization degree ${\Pi} = 4.9 {\pm} 1.1 \%$
and angle ${\Psi}= 86{\deg} {\pm} 7{\deg}$ east of north ($68\%$ confidence
level) are measured in the 2-8 keV energy range. The spectro-polarimetric
analysis shows that the polarization could be entirely due to reflection. Given
the low reflection flux in the IXPE band, this requires however a reflection
with a very large ($> 38 \%$) polarization degree. Assuming more reasonable
values, a polarization degree of the hot corona ranging from ${\sim}4$ to
${\sim}8\%$ is found. The observed polarization degree excludes a spherical
lamppost geometry for the corona, suggesting instead a slab-like geometry,
possibly a wedge, as determined via Monte Carlo simulations. This is further
confirmed by the X-ray polarization angle, which coincides with the direction
of the extended radio emission in this source, supposed to match the disc axis.
NGC4151 is the first AGN with an X-ray polarization measure for the corona,
illustrating the capabilities of X-ray polarimetry and IXPE in unveiling its
geometry.
