We identify a counterpart to a Galactic supershell in diffuse radio polarization, and use this to determine the magnetic fields associated with this object. GSH 006 - 15 + 7 has perturbed the polarized emission at 2.3 GHz, as observed in the S-band Polarization All Sky Survey (S-PASS), acting as a Faraday screen. We model the Faraday rotation over the shell, and produce a map of Faraday depth over the area across it. Such models require information about the polarized emission behind the screen, which we obtain from the Wilkinson Microwave Anisotropy Probe (WMAP), scaled from 23 to 2.3 GHz, to estimate the synchrotron background behind GSH 006 - 15 + 7. Using the modelled Faraday thickness, we determine the magnitude and the plane-of-the-sky structure of the line-of-sight magnetic field in the shell. We find a peak line-of-sight field strength of |B∥|peak=2.0+0.01 -0.7μ G. Our measurement probes weak magnetic fields in a low-density regime (number densities of ~0.4 cm-3) of the ISM, thus providing crucial information about the magnetic fields in the partially-ionized phase.