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.