  The Imaging X-ray Polarimetry Explorer (IXPE) measured with high significance
the X-ray polarization of the brightest Z-source Scorpius X-1, resulting in the
nominal 2-8 keV energy band in a polarization degree of 1.0(0.2)% and a
polarization angle of 8(6){\deg} at 90% of confidence level. This observation
was strictly simultaneous with observations performed by NICER, NuSTAR, and
Insight-HXMT, which allowed for a precise characterization of its broad-band
spectrum from soft to hard X-rays. The source has been observed mainly in its
soft state, with short periods of flaring. We also observed low-frequency
quasi-periodic oscillations. From a spectro-polarimetric analysis, we associate
a polarization to the accretion disk at <3.2% at 90% of confidence level,
compatible with expectations for an electron-scattering dominated optically
thick atmosphere at the Sco X-1 inclination of 44{\deg}; for the higher-energy
Comptonized component, we obtain a polarization of 1.3(0.4)%, in agreement with
expectations for a slab of Thomson optical depth of ~7 and an electron
temperature of ~3 keV. A polarization rotation with respect to previous
observations by OSO-8 and PolarLight, and also with respect to the radio-jet
position angle, is observed. This result may indicate a variation of the
polarization with the source state that can be related to relativistic
precession or to a change in the corona geometry with the accretion flow.
