  We present an analysis of the joint $XMM$-$Newton$ and $NuSTAR$ observations
of the radio-quiet quasar QSO B2202$-$209. Using an optical observation from
the Hale Telescope at the Palomar Observatory, we revise the redshift of the
source from the previously reported $z=1.77$ to $z=0.532$, and we estimate the
mass of the central black hole, $\log (M_{\rm BH}/M_{\odot}) = 9.08 \pm 0.18$.
The X-ray spectrum of this source can be well described by a power-law of
photon index $\Gamma = 1.82 \pm 0.05$ with $E_{\rm cut} =
152_{-54}^{+103}\,{\rm keV}$, in the rest frame of the source. Assuming a
Comptonisation model, we estimate the coronal temperature to be $kT_{\rm
e}=42\pm 3 \,{\rm keV}$ and $kT_{\rm e}= 56 \pm 3\,{\rm keV}$ for a spherical
and a slab geometry, respectively. The coronal properties are comparable to the
ones derived for local AGN, despite a difference of around one order of
magnitude in black hole mass and X-ray luminosity ($L_{2-10} = 1.93\times
10^{45}\,{\rm erg\,s^{-1}}$). The quasar is X-ray loud, with an unusually flat
observed optical-to-X-ray spectral slope $\alpha_{\rm OX} = 1.00 \pm 0.02$, and
has an exceptionally strong optical [O III] line. Assuming that both the X-ray
emission and the [O III] line are isotropic, these two extreme properties can
be explained by a nearly edge-on disk, leading to a reduction in the observed
UV continuum light.
