We present Keck/OSIRIS 1.1–1.8 \mu m adaptive optics integral field spectroscopy of the planetary-mass companion to GSC 06214-00210 , a member of the \sim 5 Myr Upper Scorpius OB association . We infer a spectral type of L0 \pm 1 , and our spectrum exhibits multiple signs of youth . The most notable feature is exceptionally strong Pa \beta emission ( EW =–11.4 \pm 0.3 Å ) which signals the presence of a circumplanetary accretion disk . The luminosity of GSC 06214-00210 b combined with its age yields a model-dependent mass of 14 \pm 2 M _ { \mathrm { Jup } } , making it the lowest-mass companion to show evidence of a disk . With a projected separation of 320 AU , the formation of GSC 06214-00210 b and other very low-mass companions on similarly wide orbits is unclear . One proposed mechanism is formation at close separations followed by planet-planet scattering to much larger orbits . Since that scenario involves a close encounter with another massive body , which is probably destructive to circumplanetary disks , it is unlikely that GSC 06214-00210 b underwent a scattering event in the past . This implies that planet-planet scattering is not solely responsible for the population of gas giants on wide orbits . More generally , the identification of disks around young planetary companions on wide orbits offers a novel method to constrain the formation pathway of these objects , which is otherwise notoriously difficult to do for individual systems . We also refine the spectral type of the primary from M1 to K7 and detect a mild ( 2- \sigma ) excess at 22 \mu m using WISE photometry .