  We report the discovery of an ultra-luminous quasar J030642.51+185315.8
(hereafter J0306+1853) at redshift 5.363, which hosts a super-massive black
hole (SMBH) with $M_{BH} = (1.07 \pm 0.27) \times10^{10}~M_\odot$. With an
absolute magnitude $M_{1450}=-28.92$ and bolometric luminosity
$L_{bol}\sim3.4\times10^{14} L_{\odot}$, J0306+1853 is one of the most luminous
objects in the early Universe. It is not likely to be a beamed source based on
its small flux variability, low radio loudness and normal broad emission lines.
In addition, a $z=4.986$ Damped Ly$\alpha$ system (DLA) with $\rm
[M/H]=-1.3\pm0.1$, among the most metal rich DLAs at $z \gtrsim 5$, is detected
in the absorption spectrum of this quasar. This ultra-luminous quasar puts
strong constraint on the bright-end of quasar luminosity function and
massive-end of black hole mass function. It will provide a unique laboratory to
the study of BH growth and the co-evolution between BH and host galaxy with
multi-wavelength follow-up observations. The future high resolution spectra
will give more insights to the DLA and other absorption systems along the
line-of-sight of J0306+1853.
