  We report spectro-polarimetric results of an observational campaign of the
bright neutron star low-mass X-ray binary Cyg X-2 simultaneously observed by
IXPE, NICER and INTEGRAL. Consistently with previous results, the broad-band
spectrum is characterized by a lower-energy component, attributed to the
accretion disc with $kT_{\rm in} \approx$ 1 keV, plus unsaturated
Comptonization in thermal plasma with temperature $kT_{\rm e} = 3$ keV and
optical depth $\tau \approx 4$, assuming a slab geometry. We measure the
polarization degree in the 2-8 keV band $P=1.8 \pm 0.3$ per cent and
polarization angle $\phi = 140^{\circ} \pm 4^{\circ}$, consistent with the
previous X-ray polarimetric measurements by OSO-8 as well as with the direction
of the radio jet which was earlier observed from the source. While polarization
of the disc spectral component is poorly constrained with the IXPE data, the
Comptonized emission has a polarization degree $P =4.0 \pm 0.7$ per cent and a
polarization angle aligned with the radio jet. Our results strongly favour a
spreading layer at the neutron star surface as the main source of the
polarization signal. However, we cannot exclude a significant contribution from
reflection off the accretion disc, as indicated by the presence of the iron
fluorescence line.
