  Radio sources at the highest redshifts can provide unique information on the
first massive galaxies and black holes, the densest primordial environments,
and the epoch of reionization. The number of astronomical objects identified at
z>6 has increased dramatically over the last few years, but previously only
three radio-loud (R2500>10) sources had been reported at z>6, with the most
distant being a quasar at z=6.18. Here we present the discovery and
characterization of P172+18, a radio-loud quasar at z=6.823. This source has an
MgII-based black hole mass of ~3x10^8 Msun and is one of the fastest accreting
quasars, consistent with super-Eddington accretion. The ionized region around
the quasar is among the largest measured at these redshifts, implying an active
phase longer than the average lifetime of the z>6 quasar population. From
archival data, there is evidence that its 1.4 GHz emission has decreased by a
factor of two over the last two decades. The quasar's radio spectrum between
1.4 and 3.0 GHz is steep (alpha=-1.31) and has a radio-loudness parameter
R2500~90. A second steep radio source (alpha=-0.83) of comparable brightness to
the quasar is only 23.1" away (~120 kpc at z=6.82; projection probability <2%),
but shows no optical or near-infrared counterpart. Further follow-up is
required to establish whether these two sources are physically associated.
