We report the discovery of a bright (
$g = 14.5$
mag (AB),
$K = 11.9$
mag (Vega)) quasar at redshift
$z=0.83$
— the optically brightest (unbeamed) quasar at
$z>0.4$
. SMSS J114447.77-430859.3, at a Galactic latitude of
$b=+18.1^{\circ}$
, was identified by its optical colours from the SkyMapper Southern Survey (SMSS) during a search for symbiotic binary stars. Optical and near-infrared spectroscopy reveals broad Mg ii, H
$\unicode{x03B2}$
, H
$\unicode{x03B1}$
, and Pa
$\unicode{x03B2}$
emission lines, from which we measure a black hole mass of
$\log_{10}\! (M_{\mathrm{BH}}/\mathrm{M}_{\odot}) = 9.4 \pm 0.5$
. With its high luminosity,
$L_{\mathrm{bol}} = (4.7\pm1.0)\times10^{47}\,\mathrm{erg\,s}^{-1}$
or
$M_{i}(z=2) = -29.74$
mag (AB), we estimate an Eddington ratio of
$\approx1.4$
. As the most luminous quasar known over the last
${\sim}$
9 Gyr of cosmic history, having a luminosity
$8\times$
greater than 3C 273, the source offers a range of potential follow-up opportunities.