  We report on a comprehensive analysis of simultaneous X-ray polarimetric and
spectral data of the bright atoll source GX 9+9 with the Imaging X-ray
Polarimetry Explorer (IXPE) and NuSTAR. The source is significantly polarized
in the 4--8 keV band, with a degree of $2.2\% \pm 0.5\%$ (uncertainty at the
68% confidence level). The NuSTAR broad-band spectrum clearly shows an iron
line, and is well described by a model including thermal disk emission, a
Comptonized component, and reflection. From a spectro-polarimetric fit, we
obtain an upper limit to the polarization degree of the disk of 4% (at 99%
confidence level), while the contribution of Comptonized and reflected
radiation cannot be conclusively separated. However, the polarization is
consistent with resulting from a combination of Comptonization in a boundary or
spreading layer, plus reflection off the disc, which gives a significant
contribution in any realistic scenario.
