We present the discovery of a companion near the deuterium-burning mass limit located at a very wide distance, at an angular separation of 4.6''+/-0.1'' (projected distance of ~ 670 AU) from UScoCTIO 108, a brown dwarf of the very young Upper Scorpius association. Optical and near-infrared photometry and spectroscopy confirm the cool nature of both objects, with spectral types of M7 and M9.5, respectively, and that they are bona fide members of the association, showing low gravity and features of youth. Their masses, estimated from the comparison of their bolometric luminosities and theoretical models for the age range of the association, are 60+/-20 and 14+2-8 MJup, respectively. The existence of this object around a brown dwarf at this wide orbit suggests that the companion is unlikely to have formed in a disk based on current planet formation models. Because this system is rather weakly bound, they probably did not form through dynamical ejection of stellar embryos.