  A brief Chandra observation of the ultraluminous quasar, SDSS
J010013.02+280225.8 at redshift 6.326, showed it to be a relatively bright,
soft X-ray source with a count rate of about 1 ct/ks. In this paper we present
results for the quasar from a 65ks XMM-Newton observation, which well
constrains its spectral shape. The quasar is clearly detected with a total of
$\sim$ 460 net counts in the 0.2-10 keV band. The spectrum is characterised by
a simple power-law model with photon index of $\Gamma = 2.30^{+0.10}_{-0.10}$,
and the intrinsic 2-10 keV luminosity is $3.14\times10^{45}$ erg
$\text{s}^{-1}$. The 1 $\sigma$ upper limit to any intrinsic absorption column
density is $N_{H} = 6.07\times 10^{22} {\text{cm}}^{-2}$. No significant iron
emission lines were detected. We derive the X-ray-to-optical flux ratio
$\alpha_{\text{ox}}$ of $-1.74\pm$0.01, consistent with the values found in
other quasars of comparable ultraviolet luminosity. We did not detect
significant flux variations either in the XMM-Newton exposure or between
XMM-Newton and XMM-Newton observations, which are separated by $\sim$ 8 months.
The X-ray observation enables the bolometric luminosity to be calculated after
modelling the spectral energy distribution: the accretion rate is found to be
sub-Eddington.
