We present Gemini near-infrared adaptive optics imaging and spectroscopy of a planetary-mass candidate companion to 1RXS J160929.1–210524, a roughly solar-mass member of the 5 Myr old Upper Scorpius association. The object, separated by 2.22″ or 330 AU at ~150 pc, has infrared colors and spectra suggesting a temperature of 1800₋₁₀₀⁺²⁰⁰ K, and spectral type of L4₋₂⁺¹. TheH- andK-band spectra provide clear evidence of low surface gravity, and thus youth. Based on the widely used DUSTY models, we infer a mass of 8₋₂⁺⁴M_Jup. If gravitationally bound, this would be the lowest mass companion imaged around a normal star thus far, and its existence at such a large separation would pose a serious challenge to theories of star and planet formation.