  This paper reports the first detection of polarization in the X-rays for
atoll-source 4U 1820-303, obtained with the Imaging X-ray Polarimetry Explorer
(IXPE) at 99.999% confidence level (CL). Simultaneous polarimetric measurements
were also performed in the radio with the Australia Telescope Compact Array
(ATCA). The IXPE observations of 4U 1820-303 were coordinated with Swift-XRT,
NICER, and NuSTAR aiming to obtain an accurate X-ray spectral model covering a
broad energy interval. The source shows a significant polarization above 4 keV,
with a polarization degree of 2.0(0.5)% and a polarization angle of -55(7) deg
in the 4-7 keV energy range, and a polarization degree of 10(2)% and a
polarization angle of -67(7) deg in the 7-8 keV energy bin. This polarization
also shows a clear energy trend with polarization degree increasing with energy
and a hint for a position-angle change of about 90 deg at 96% CL around 4 keV.
The spectro-polarimetric fit indicates that the accretion disk is polarized
orthogonally to the hard spectral component, which is presumably produced in
the boundary/spreading layer. We do not detect linear polarization from the
radio counterpart, with a 99.97% upper limit of 50% at 7.25 GHz.
