  We present the discovery of a substellar companion on a wide orbit around the
~2.5 Msun star HIP 78530, which is a member of the 5 Myr-old Upper Scorpius
association. We have obtained follow-up imaging over two years and show that
the companion and primary share common proper motion. We have also obtained JHK
spectroscopy of the companion and confirm its low surface gravity, in
accordance with the young age of the system. A comparison with Drift-Phoenix
synthetic spectra indicates an effective temperature of 2800+/-200 K and a
comparison with template spectra of young and old dwarfs indicates a spectral
type of M8+/-1. The mass of the companion is estimated to be 19-26 Mjup based
on its bolometric luminosity and the predictions of evolutionary models. The
angular separation of the companion is 4.5", which at the distance of the
primary star, 156.7 pc, corresponds to a projected separation of ~710 AU. This
companion features one of the lowest mass ratios (~0.009) of any known
companion at separations greater than 100 AU.
