  Magnetospheric processes seen in gas-giants such as aurorae and
circularly-polarized cyclotron maser radio emission have been detected from
some brown dwarfs. However, previous radio observations targeted known brown
dwarfs discovered via their infrared emission. Here we report the discovery of
BDR J1750+3809, a circularly polarized radio source detected around 144 MHz
with the LOFAR telescope. Follow-up near-infrared photometry and spectroscopy
show that BDR J1750+3809 is a cold methane dwarf of spectral type T$6.5\pm 1$
at a distance of $65^{+9}_{-8}\,{\rm pc}$. The quasi-quiescent radio spectral
luminosity of BDR J1750+3809 is $\approx 5\times 10^{15}\,{\rm erg}\,{\rm
s}^{-1}\,{\rm Hz}^{-1}$ which is over two orders of magnitude larger than that
of the known population of comparable spectral type. This could be due to a
preferential geometric alignment or an electrodynamic interaction with a close
companion. In addition, as the emission is expected to occur close to the
electron gyro-frequency, the magnetic field strength at the emitter site in BDR
J1750+3809 is $B\gtrsim 25\,{\rm G}$, which is comparable to planetary-scale
magnetic fields. Our discovery suggests that low-frequency radio surveys can be
employed to discover sub-stellar objects that are too cold to be detected in
infrared surveys.
