  We detect a high level of polarization in the X-ray emission of the
black-hole binary 4U 1630-47 in an observation with the Imaging X-ray
Polarimetry Explorer. The 2-8 keV polarization degree is 8 % at a position
angle of $18^\circ$, with the polarization degree increasing significantly with
energy, from ~6 % at ~2 keV to ~ 11 % at ~8 keV. The continuum emission in the
spectrum of simultaneous observations with the Neutron Star Interior
Composition Explorer, NICER, is well described with only a thermal disc
spectrum, with stringent upper limits to any Comptonized emission from the
corona. Together with the lack of significant variability in the Fourier power
spectrum, this suggests that the source was in the high-soft state at the time
of these observations. The NICER spectrum reveals the presence of several
absorption lines in the 6-9 keV band that we fit with two ionized absorbers,
providing evidence of the presence of a strong disk wind, which supports the
idea that the source was in the soft state. Previous measurements of X-ray
polarization in other sources in harder states were associated with the corona
or the jet in those systems. Given that the corona is significantly absent in
this observation of 4U 1630-47, and that the jet in black-hole binaries is
quenched in the high-soft state, we speculate that in this observation of 4U
1630-47, the polarization likely arises from the direct and reflected radiation
of the accretion disk in this source.
