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 $3.14times10^{45}$ erg $text{s}^{-1}$. The 1 $sigma$ upper limit to any intrinsic absorption column density is $N_{H} = 6.07times 10^{22} {text{cm}}^{-2}$. No significant iron emission lines were detected. We derive the X-ray-to-optical flux ratio $alpha_{text{ox}}$ of $-1.74pm$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.
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